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BepiColombo Mission

Spacecraft     MPO    Status     Launch    MMO    MTM    Ground Segment    References

BepiColombo is one of ESA's (European Space Agency) cornerstone missions being conducted in cooperation with Japan, it will explore Mercury, the planet closest to the Sun. Europe's space scientists have identified the mission as one of the most challenging long-term planetary projects, because Mercury's proximity to the Sun makes it difficult for a spacecraft to reach the planet and to survive in the harsh environment found there. The scientific interest in going to Mercury lies in the valuable information that such a mission can provide to enhance our understanding of the planet itself as well as the formation of our Solar System; information which cannot be obtained with observations made from Earth. The overall goal is to study and understand the composition, geophysics, atmosphere, magnetosphere and history of Mercury, the least explored planet in the inner Solar System. In particular, the mission has the following scientific objectives: 1) 2) 3) 4) 5) 6) 7) 8)

- Investigate the origin and evolution of a planet close to the parent star

- Study Mercury as a planet: its form, interior structure, geology, composition and craters

- Examine Mercury's vestigial atmosphere (exosphere): its composition and dynamics

- Probe Mercury's magnetized envelope (magnetosphere): its structure and dynamics

- Determine the origin of Mercury's magnetic field

- Investigate polar deposits: their composition and origin

- Perform a test of Einstein's theory of general relativity

Set to arrive at Mercury in 2024, BepiColombo will investigate properties of the innermost planet of our Solar System that are still mysterious, such as its high density, the fact that it is the only planet with a magnetic field similar to Earth's, the much higher than expected amount of volatile elements detected by NASA's Messenger probe and the nature of water ice that may exists in the permanently shadowed areas at the poles.

The BepiColombo mission is named after Professor Giuseppe (Bepi) Colombo (1920-1984) from the University of Padova, Italy, a mathematician and engineer of astonishing imagination. He was the first to see that an unsuspected resonance is responsible for Mercury's habit of rotating on its axis three times for every two revolutions it makes around the Sun. He also suggested to NASA how to use a gravity-assist swing-by of Venus to place the Mariner 10 spacecraft in a solar orbit that would allow it to fly by Mercury three times in 1974-5.

ESA's Science Program Committee decided at its meeting in Naples in 1999 to name the Mercury cornerstone mission in honor of Giuseppe Colombo's achievements.

Mercury is small compared to the Earth, with a diameter of only 4878 km. It orbits the Sun in an elliptic orbit between 0.3 and 0.47 AU from the Sun. Mercury is difficult to observe from the Earth, due to its close proximity to the very bright Sun. For an in-depth study of the planet and its environment, it is therefore necessary to operate a spacecraft equipped with scientific instrumentation around the planet. It is, however, difficult for a spacecraft to reach Mercury, as even more energy is needed than sending a mission to Pluto. Departing from Earth, a spacecraft needs to decelerate to come closer to the Sun and as the solar gravitational force increases with the square of the distance, the required reverse thrust increases accordingly. Furthermore, the thermal environment close to the Sun and close to the hottest planet in the solar system is extremely aggressive, as the direct solar radiation is 10 times higher than at Earth's distance. 9)

Despite the advances in space flight and the growth in planetary research over the last few decades, enabling detailed investigations of the Earth, Mars, Venus, the outer planets and several moons and asteroids, scientists have not been able to observe much of Mercury. For a very long time the data delivered by NASA's (US National Aeronautics and Space Administration) Mariner 10, which visited Mercury in1974-1975, was among the best available. During these flybys Mariner 10 was able to image about 45% of the planet's surface and to discover its unexpected magnetic field. Further discoveries by Mariner 10 are the existence of gaseous species forming an exosphere and the presence of a unique magnetosphere. However, little to nothing is known about Mercury's interior structure or its elemental and mineralogical composition.

With the launch of the NASA Discovery class mission MESSENGER (Mercury Surface, Space ENvironment, Geochemistry and Ranging) in 2004, the first spacecraft was launched to orbit Mercury. MESSENGER already collected data from two Venus flybys and three Mercury flybys in 2008 and 2009. The orbital phase of the MESSENGER mission started in March 2011. The MESSENGER data will provide valuable discoveries of Mercury and its environment that can be used by the BepiColombo mission in tuning its observations to the most important investigations of planet Mercury.

NASA's MESSENGER mission came to a planned end on May 30, 2015 when it slammed into Mercury's surface at about 14,000 km/h and created a new crater on the planet's surface. MESSENGER ended up exceeding its planned mission timeline by three years, by which time the spacecraft had completely depleted its fuel. The last of the fuel was used to position it within the gravitational pull of Mercury and the Sun, so it could delay as long as possible its inevitable plummet towards the surface – while continuing to beam back images – and go out with a bang. 10)

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Figure 1: Messenger's iridescent Mercury (image credit: NASA, JHU/APL, Carnegie Institution, Washington) 11)

Legend to Figure 1: The contrasting colors have been chosen to emphasize the differences in the composition of the landscape across the planet. The darker regions exhibit low-reflectance material, particularly for light at redder wavelengths. As a result, these regions take on a bluer cast.

- The crisscrossing streaks across the disc of the planet show up in shades of light blue, grey and white. These regions take on a light blue hue for a different reason: their youthfulness. As material is exposed to the harsh space environment around Mercury it darkens, but these pale ‘rays' are formed from material excavated from beneath the planet's surface and sent flying during comparatively recent impacts. For this reason, they have retained their youthful glow.

- The yellowish, tan-colored regions are "intermediate terrain". Mercury also hosts brighter and smoother terrain known as high-reflectance red plains. One example can be seen towards the upper right, where there is a prominent patch that is roughly circular. This is the Caloris basin, an impact crater thought to have been created by an asteroid collision during the Solar System's early days.

As the nearest planet to the Sun, Mercury has an important role in showing us how planets form. Mercury, Venus, Earth and Mars make up the family of terrestrial planets; each one carrying essential information to trace the history of the whole group.

The knowledge of how they originated and evolved is key to understanding how conditions supporting life arose in the Solar System, and possibly elsewhere. As long as Earth-like planets orbiting other stars remain inaccessible to astronomers, the Solar System is the only laboratory where scientists can test models applicable to other planetary systems.

Exploring Mercury is therefore fundamental to answering important astrophysical and philosophical questions such as 'Are Earth-like planets common in the Galaxy?'

A European mission to Mercury was first proposed in May 1993. Although an assessment showed it to be too costly for a medium-size mission, ESA made a Mercury orbiter one of its three new cornerstone missions when the Horizon 2000 science program was extended in 1994. Gaia competed with BepiColombo for the fifth cornerstone mission. In October 2000, ESA approved a package of missions for 2008–2013 and both BepiColombo and Gaia were approved.

In February 2007, the mission was approved as part of the Cosmic Vision program. Following an unavoidable increase in the mission's mass during 2008, the launch vehicle was changed from Soyuz-Fregat to Ariane 5. Final approval for the redesigned mission was given by ESA's Science Program Committee in November 2009.

BepiColombo represents the first time ESA and JAXA have joined forces for the implementation of a major space science mission.

BepiColombo's mission is especially challenging because Mercury's orbit is so close to our star, the Sun. The planet is hard to observe from a distance, because the Sun is so bright. Furthermore, it is difficult to reach because a spacecraft must lose a lot of energy to ‘fall' towards the planet from the Earth. The Sun's enormous gravity presents a challenge in placing a spacecraft into a stable orbit around Mercury.

Only NASA's Mariner 10 and Messenger missions have visited Mercury so far. Mariner 10 provided the first-ever close-up images of the planet when it flew past three times in 1974-1975. En route to its final destination in orbit around Mercury in 18 March 2011, Messenger flew past the planet 3 times (14 January 2008, 6 October 2008, and 29 September 2009), providing new data and images. Once BepiColombo arrives in 2024, it will help reveal information on the composition and history of Mercury. It should discover more about the formation and the history of the inner planets in general, including Earth.

Mercury is a major Roman god. He is the patron god of financial gain, commerce, eloquence (and thus poetry), messages/communication (including divination), travelers, boundaries, luck, trickery and thieves; he is also the guide of souls to the underworld. In Greek mythology, Hermes is an Olympian god of transitions and boundaries. In the Roman adaptation of the Greek pantheon, Hermes is identified with the Roman god Mercury. — Hence, in an orbit around the planet Mercury, the point that is closest to Mercury is termed "periherm" while the farthest point of a spacecraft orbit is called "apoherm".

Since Mercury is the closest planet to the sun (0.31 AU to 0.47 AU distant) a peak solar intensity of 11 solar constants (14,500 W/m2) is experienced which imposes enormous thermal challenges on the spacecraft modules and their external equipment.

Table 1: Some history and background 12)

The space segment design is driven essentially by the scientific payload requirements,the launch mass constraints and the harsh thermal and radiation environment at Mercury. Key technologies required for the implementation of this challenging mission include the following:

• High-temperature thermal control materials (coatings, adhesives,resins,MLI,OSR).

• Radiator design for high-infrared environment.

• High-temperature and high-intensity solar cells,diodes and substrates for the solar arrays.

• High-temperature steerable high-gain and medium-gain antennas.

• High specific impulse(Isp=4300 s)and high total impulse (23.7 mNs),to be provided by gridded ion engines.

• Payload technology,such as detectors,filters and laser technology.

 



Space segment:

The BepiColombo mission is based on two spacecraft:

1) MPO (Mercury Planetary Orbiter) to map the planet. MPO is a three-axis stabilized and a nadir pointing spacecraft with an instrument suite of 11 experiments and instruments. MPO is led by ESA. The MPO will focus on a global characterization of Mercury through the investigation of its interior, surface, exosphere and magnetosphere. In addition, it will test Einstein's theory of general relativity.

2) MMO (Mercury Magnetospheric Orbiter) to investigate its magnetosphere. MMO is a spinning spacecraft carrying a payload of five experiments and instruments. The MMO is led by JAXA.

Among several investigations, BepiColombo will make a complete map of Mercury at different wavelengths. It will chart the planet's mineralogy and elemental composition, determine whether the interior of the planet is molten or not, and investigate the extent and origin of Mercury's magnetic field.

MPO is ESA's scientific contribution to the mission. JAXA/ISAS (Japan Aerospace Exploration Agency/Institute of Space and Astronautical Science) is providing the MMO. ESA is also building the MTM (Mercury Transfer Module), which will carry the two orbiters to their destination, and the MOSIF (MMO Sunshield and Interface Structure), which provides thermal protection and the mechanical and electrical interfaces for the MMO. The MCS (Mercury Composite Spacecraft) consists of the MPO, MMO, MTM and MOSIF. ESA is responsible for the overall mission design, the design, development and test of the MPO, MTM and MOSIF, the integration and test of the MCS and the launch. 13)

Parameter

MPO (Mercury Planetary Orbiter)

MMO (Mercury Magnetospheric Orbiter)

Stabilization

3-axis stabilized

15 rpm spin-stabilized

Orientation

Nadir pointing

Spin axis at 90º to Sun

Orbit at Mercury

Polar orbit, period of 2.3 hr, 480 x 1500 km

Polar orbit, period of 9.3 hr, 590 x 11,640 km

Spacecraft mass

4100 kg (total mass at launch)
1150 kg (in Mercury orbit)


275 kg (in Mercury orbit)

Spacecraft size

3.9 x 2.2 x 1.7 m (excluding solar wings)

1.9 m ∅ x 1.1 m

Payload mass, power

80 kg, 100-150 W

45 kg, 90 W

Telemetry band

X/Ka-band

X-band

Data volume (downlink)

1550 Gbit/year

160 Gbit/year

Equivalent average data rate

50 kbit/s

5 kbit/s

Antenna

High-temperature resistant 1.0 m X/Ka-band high-gain steerable antenna

0.8 m X-band phased array high-gain antenna

Operational lifetime at Mercury

> 1 year

> 1 year

Table 2: Key parameters of the two spacecraft

Launch, journey and orbit:

The BepiColombo trajectory employs a solar electric propulsion system so that a combination of low-thrust arcs and flybys at Earth, Venus and Mercury are used to reach Mercury with low relative velocity. A brief summary of the key stages in the journey to Mercury are given here:

• Launch on Ariane 5 in April 2018 on escape trajectory to reach Venus

• Cruise trajectory with solar electric propulsion stage - the SEPM (Solar Electric Propulsion Module), up to 290 mN thrust - plus six gravity assists: Venus (twice) and Mercury (four times)

• Approximately 6.7 year cruise phase to Mercury

• Ion propulsion stage jettisoned shortly before arrival at Mercury

• Capture and insertion by chemical propulsion engines within the MPO

• On reaching MMO orbit the MMO is released

• MOSIF is released before further descending to the MPO orbit

• MPO is inserted into final orbit using thrust from chemical propulsion engines

• For MPO and MMO: one Earth-year (4 Mercury years) operations in Mercury orbit with optional one year extension.

Key mission dates

Event

April 2018

Launch

25 July 2019

First Venus flyby

20 May 2020

Second Venus flyby

09 April 2021

First Mercury flyby

27 March 2022

Second Mercury flyby

16 December 2023

Third Mercury flyby

24 January 2024

Fourth Mercury flyby

18 December 2024

Arrival at Mercury

27 March 2015

MPO in final orbit

01 May 2026

End of nominal mission

01 May 2027

End of extended mission

Table 3: Key mission dates for a 2018 launch into a heliocentric transfer orbit

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Figure 2: Artist's rendition of the BepiColombo MCS (Mercury Composite Spacecraft) in cruise configuration heading toward Mercury (image credit: ESA) 14) 15)

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Figure 3: Artist's rendition of BepiColombo's MPO and MMO spacecraft in their respective Mercury orbits (image credit: ESA, C. Carreau)

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Figure 4: Artist's view of the BepiColombo spacecraft MPO (ESA, foreground) and MMO (JAXA, background) at Mercury (image credit: ESA/ATG medialab. The Mercury image was taken by NASA's Messenger spacecraft, image credit: NASA, JHU/APL, and Carnegie Institution) 16)

 

Spacecraft

Industrial involvement:

• In November 2006, ESA awarded the prime contract for the Implementation Phase to Airbus Defence and Space, former EADS Astrium of Friedrichshafen, Germany. The PDR (Preliminary Design Review) was completed in October 2008.

This included the design and procurement of the 'cruise-composite' spacecraft, including the ESA's MPO (Mercury Planetary Orbiter), the MTM (Mercury Transfer Module), the MMO's sunshield and the interface between the MPO and the MMO. 17) Furthermore, the prime contractor provides the design and development of the data management and attitude and orbit control subsystems, and the integration of the engineering model (Ref. 3).

• In December 2012, TAS-I (Thales Alenia Space-Italia) signed a contract with Astrium GmbH for BepiColombo. TAS-I is part of the industrial Core Team, coordinating 35 European manufacturers within its workpackage. The contract concerns the telecommunications, thermal control and electric power distribution systems, along with satellite integration and testing, plus support during the launch campaign. In addition, TAS-I is developing the X- and Ka-band transponders, onboard computer, mass memory and high-gain antenna, a 1.1 m diameter dish antenna that will enable the satellite to communicate with Earth, while also carrying out a Radio Science experiment during the mission. 18)

• In the UK, Airbus DS (formerly Astrium Ltd.) is the co-prime contractor for the electrical and chemical propulsion systems, for the structure of all modules and for the thermal control of the MTM (Mercury Transfer Module). Airbus DS in France will develop the onboard software.

• The MMO and its scientific payload are designed and developed by JAXA. They are responsible for procuring the spacecraft from an industrial team led by NEC.

MCS (Mercury Composite Spacecraft):

The composite spacecraft, as shown in Figure 5, consists of four modules: the MPO (Mercury Planetary Orbiter), the MMO (Mercury Magnetospheric Orbiter) protected by the MOSIF (MMO SunShield and InterFace Structure) and the MTM (Mercury Transfer Module). The MPO,developed by ESA, is the scientific module characterized by a set of observation instruments to study the surface of Mercury and the gravity field of the planet. The MMO, developed by JAXA, is a spin stabilized scientific module aimed at the study of the magnetic field of Mercury. Both orbiters, the MPO and the MMO, will be carried on top of the transfer module MTM during the cruise phase. 19) 20) 21)

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Figure 5: Schematic view of the BepiColombo MCS (Mercury Composite Spacecraft), image credit: Airbus DS

As the mission evolves, then the number of modules decreases. These evolving configurations are a composite of x modules, hence the various configurations are known as MCSn (Mercury Composite Spacecraft), where "n" represents the states L for launch, C for Cruise, A for Approach and O for Orbit.

The MPO, whatever tasks it may perform during cruise, is ultimately a free-flying spacecraft containing all the capabilities needed to perform its scientific mission – for which careful optimization was necessary when considering the thermal environment. The MPO therefore contains most of the capabilities also needed during cruise. In order not to compromise the MPO design by taking unnecessary hardware into Mercury orbit, hardware needed solely for cruise is accommodated in a separate MTM (Mercury Transfer Module).

The MMO (Mercury Magnetospheric Orbiter) is eventually also a free-flying spacecraft containing all the capabilities needed to perform its scientific mission. However with the spacecraft capabilities controlled from the MPO during cruise, the MMO then remains passive throughout (apart from periodic check-outs). Since the MMO is a normally spinning spacecraft, it requires thermal protection during the 3-axis stabilized cruise.

The 4th module of the MCS derives from the MMO's needs: the MOSIF (MMO SunShield and InterFace Structure) providing thermal protection as well as all the interfaces between MPO and MMO.

The MCSL (Mercury Composite Spacecraft -Launch) and MCSC (Mercury Composite Spacecraft-Cruise) are composed of:

1) MPO (Mercury Planetary Orbiter)

- Spacecraft optimized for its operational mission

- Performs command & control for MCS (with only minor hardware modification for MCS configurations, notably the size of the reaction wheels to control the MCS)

- CPS (Chemical Propulsion System) not used during cruise, however after MTM separation the MPO performs approach propulsion and apoherm lowering of Mercury orbit.

2) MMO (Mercury Magnetospheric Orbiter)

- Spins during operational mission

- Is passive during cruise – apart from checkouts.

3) MOSIF (MMO SunShield and InterFace Structure)

- Thermal protection for the MMO

- Mechanical interface for the MMO

- Harness routing between MPO and MMO

4) MTM (Mercury Transfer Module)

- Provides MEPS (MTM Electric Propulsion System) plus chemical propulsion (for cruise AOCS and navigation correction)

- Provides power for electric propulsion system and for MPO +MMO

- Separated before capture into Mercury orbit.

The MCSA (Mercury Composite Spacecraft-Approach) is created upon MTM separation. On reaching the MMO orbit, the MMO is released to create the MCSO (Mercury Composite Spacecraft-Orbit). The MOSIF is ejected shortly afterwards to leave the MPO.

Power Subsystem: Each of the 3 modules MPO, MMO and MTM contains power generation, storage and distribution hardware. The MPO and MMO power subsystems supply standard 28 V regulated power within the module, with the MMO being supplied by the MPO, as long as both modules are connected. During the cruise phase the power subsystem is a composite of all 3 modules (controlled from the MPO) whereby, all power is provided by the MTM solar array. The MTM generates both 100 V and 28 V supplies. 22)

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Figure 6: Composition of MCS (Mercury Composite Spacecraft), showing module functions and contributions to MCS (image credit: Airbus DS)

 



MPO (Mercury Planetary Orbiter) Spacecraft

The MPO design is optimized to meet the needs of the payload when the spacecraft is in its operational orbit. The payload components are mounted on the nadir side of the spacecraft, with certain instruments or sensors located directly at the main radiator, to achieve low detector temperatures (Ref. 13).

Structure: The spacecraft structure uses a double-H configuration, designed to harmonize with the single radiator plane necessitated by the Mercury orbit. Heat generated by spacecraft subsystems and payload components, as well as heat that is coming from the Sun and Mercury as it "leaks" through the blankets into the spacecraft, is carried to the radiator by panel-embedded heat pipes. The structural design provides free access to all equipment and instruments during the AIT (Assembly, Integration and Test) program. The design is mass efficient, with the primary structure serving as the mounting surface for all equipment; it will remain permanently assembled during AIT, avoiding the need for connector brackets and preventing alignment disturbances. MPO has four-point bolted interfaces to both the MTM (Mercury Transfer Module) and the MOSIF (MMO Sunshield and Interface Structure), which provides thermal protection and the mechanical and electrical interfaces for the MMO during the journey to Mercury.

The configuration and thermal design provide a classical thermal environment for internally mounted instrument equipment – avoiding costly development programs by re-use of available hardware – while employing dedicated high temperature technologies for external items such as antennas, the solar array, the sun sensors and MLI (Multi-Layer Insulation), which are exposed to the harsh thermal environment around Mercury.

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Figure 7: MPO – showing equipment panel perpendicular to radiator (image credit: Airbus DS)

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Figure 8: MPO spacecraft in deployed configuration (image credit: Airbus DS)

Power system generation: The MPO provides a 28 V regulated bus which also feeds the MMO during Cruise. The MPO includes a 96 Ah lithium-ion battery. The SA (Solar Array) uses both OSRs (Optical Solar Reflectors) and the control of the sun incidence angle to maintain the temperature below 190ºC. For most of the Mercury year, the solar array requires continuous rotation, in order to generate adequate power while at the same time limiting the temperature. The three-panel array has its rotation axis in an optimized direction, nevertheless, the so-called "artificial eclipses" (a condition in which the Sun vector is along the rotational axis of the solar array, i.e. no power is available) occur for short periods at certain times of the Mercury year due to the solar array mounting geometry. The solar array will provide up to 1000 W of electrical power during full science operations phases. During both the natural and artificial eclipses the battery in the MPO will provide electrical energy to the spacecraft in order to allow the scientific operations to continue without interruption (Ref. 22).

Solar Array Control: Because of the intense heat, the single-sided MPO solar array features a mix of solar cells and Optical Surface Reflectors (OSR) to keep its temperature below 200°C. The large MTM solar arrays (area of over 40 m2 in total) use the same high-temperature technology and can provide up to 13 kW power. During cruise, the entire composite is powered through the MTM SA, while the MPO SA is only required at Mercury (remaining edge on to the sun during cruise to limit degradation).

Both arrays can be rotated around their longitudinal axis using dedicated solar array drive mechanisms, under control of the AOCS. To maintain the temperature in the allowed range, both arrays require a special control approach, commanding an offpointing while still achieving sufficiently high power generation.

MTM solar array control in cruise: Down to a sun distance of 0.62 AU, the MTM solar array can be pointed straight at the sun with no thermal limitations. At sun distances smaller than that, the array must also be offpointed to not violate maximum operating temperatures.

MPO solar array control at Mercury: Due to Mercury albedo and infrared radiation, the maximum exposure of the MPO solar array towards the sun varies over the MPO operational orbit. The MPO SA hence has to be rotated continuously to avoid violation of temperature limits.

AOCS (Attitude and Orbit Control Subsystem): The AOCS equipment consists of:

• Three STRs (Star Trackers), each comprising a star tracker unit housing the optics and electronics, a shutter - which can be closed in the event of a major attitude control anomaly - and a baffle

• Two IMUs (Inertial Measurement Units), including four high-accuracy rate-integrating gyros and four accelerometers in a tetrahedral configuration, together with the processing electronics

• Two redundant sets of two FSS (Fine Sun Sensors)

• Four reaction wheel assemblies, controlled by two sets of wheel drive electronics

• Two redundant sets of four 22 N hydrazine / MON-3 (Mixed Oxides of Nitrogen), a mixture of nitrogen tetroxide and 3% of nitric oxide thrusters to provide the change in velocity (ΔV) needed for orbit capture and orbit lowering to the MMO and MPO operational orbits

• Two redundant sets of four 10 N monopropellant (hydrazine) thrusters for attitude control and reaction wheel momentum off-loading.

The reaction wheels are mounted in a tetrahedral configuration; attitude control can be achieved with four wheels operating simultaneously (the nominal operational scenario) or any combination of three wheels.

During science operations, at least two STRs will be used in combination. In the event of major system anomaly on the spacecraft and consequent loss of attitude control, dedicated shutters will protect the STR optical paths to prevent damage due to accidental sun pointing.

For MPO science operations the AOCS must provide continuous nadir pointing whilst meeting accuracy and stability requirements. Two Star Trackers (plus a 3rd for redundancy) and an IMU are co-mounted with instruments on an optical bench while 4 reaction wheels serve as actuators (with 5 N thrusters used for wheel offloading). The AOCS also controls the thermally critical orientation of the solar array and the 22 N thrusters for orbit maneuvers during MOI (Mercury Orbit Insertion).

This basic AOCS is enhanced with sun sensors for survival mode and is further enhanced for the MCS configuration when the MEPS thrusters and MTM 10 N thrusters serve as actuators. As for the MPO, MTM Solar Arrays are also thermally controlled by the appropriate orientation.

During the cruise phase, the AOCS controls the MEPS thruster orientation and corresponding MCS attitude as required by the uploaded mission timeline – with fine pointing of the MEPS thrusters minimizing momentum accumulation by the reaction wheels.

The thermal environment experienced in the MPO orbit and during cruise allows (for a number of thermally critical items) only deviations from nominal attitudes in the order of seconds before overheating and damage occurs. In the event of an OBC reboot, the Survival Mode will be entered and the AOCS control will be transferred to the FCE and a second IMU. In Survival Mode the AOCS uses Sun sensors as the attitude reference. Different survival attitudes apply for the various spacecraft configurations.

From the many changes of flight configuration, the number of actuators employed and the stringent safe and survival modes the AOCS consists of 17 operational modes.

AOCS operations in particular are impacted by S/C modularity. Preparations of attitude slew or orbit control maneuvers have to take into account the vastly different S/C characteristics. Different AOCS guidance contexts need to be maintained depending on the S/C configuration. For instance, the S/C attitude when entering safe mode is configuration-dependent (Figure 9). AOCS solar array guidance is entirely different between MCSC and MPO/MCSA/MCSO configurations. As a result, the interface between the Flight Control Team and the Flight Dynamics team for commanding the S/C is particularly sophisticated.

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Figure 9: Different safe mode attitudes depending on S/C configuration: +Y sun pointing for MCSC, sun close to +X for MCSA/O, sun close to –Z for MPO. Safe mode concept is to have a rotation around the sun line, which has to be in synch with the orbital motion around Mercury in MPO and MCSO configurations (image credit: ESA)

TCS (Thermal Control Subsystem): The MPO TCS must regulate the equipment temperatures (achieving standard equipment levels), transfer heat to the single radiator, shield the radiator from planet infrared illumination, reject 1200 W of dissipated heat from the payload and spacecraft equipments and reject up to 300 W of parasitic heat which enters the MPO body. These functions are achieved by means of (Ref. 21):

- Heatpipes embedded in the equipment mounting panels to collect and transfer the heat the radiator panel

- Spreader heatpipes in the radiator panel, thermally connected to the equipment panels by 90° linking heatpipes

- 97 heatpipes are used, of which a few are 3-dimensional hence difficult to test on ground

- Fixed louvers are mounted in front of the radiator to reflect the planet infrared radiation away from the radiator whilst allowing the radiator an extensive view to space

- The entire MPO body is covered with high temperature MLI developed for BepiColombo

- The outer heat shield comprises 2 layers of Nextel ceramic cloth followed by 11 aluminum layers. The Nextel layers reach 380°C

- Moving inwards to lower temperature, 26 layers of aluminized Upilex are followed by 10 layers of aluminized Mylar

- Spacers of glass fiber and AAerofoam are used to separate the layers in the 4 packets, while Kapton rosettes separate the packets

- The installed MLI has a thickness of 65 mm. The total MLI mass is 94 kg.

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Figure 10: Photo of a section through the MPO MLI (image credit: Airbus DS)

The MOSIF MLI must shade the MMO and limit the infrared heat load to the MMO. The MOSIF MLI is characterized by:

- A single Nextel outer layer

- 7 dimpled titanium layers separated by glass spacers

- It is freely supported over lengths of up to 2.5 m and must withstand the vibration and acoustic environments of the launch.

The MTM TCS must regulate the equipment temperatures, distribute heat in the radiators, reject 2000 W of dissipated heat equipments and reject up to 300 W of parasitic heat which enters the MTM body. These functions are achieved by means of:

- Heatpipes embedded in the radiator panels (which also serve for equipment mounting)

- The embedded heatpipe network is enhanced by surface heatpipes

- 63 heatpipes are used

- Derivatives of the high-temperature MLI are used.

Further MLI applications result from the stack configuration and the separation interfaces of the MPO:

- While the modules are protected as described above, solar illumination gaps between modules can not be tolerated

- Elaborate Gap Closure MLI is implemented between MTM-MPO and MPO-MOSIF. This MLI is in contact with the MPO and is attached to the separating modules (Figure 20).

- The 4-point mechanical interfaces between modules leave holes of Ø140 mm in MLI (to these add 2 x Ø170 mm holes for the connectors at each interface). These holes are closed by DTCs (Deployable Thermal Covers) containing MLI disks to drastically reduce the heat load. The 12 DTCs are mounted between the MLI layers with the cylinder and ring (white coated) protruding through the MLI heatshield.

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Figure 11: Photo of a DTC (Deployable Thermal Cover) to close separation apertures (image credit: Airbus DS, Ref. 21)

DHS (Data Management System): The basic MPO DHS comprises redundant OBCs (On-Board Computer) and an internally redundant SSMM (Solid State Mass Memory) for payload and spacecraft data storage. A MIL-STD-1553B bus is used for spacecraft telemetry and telecommand while all payload TM/TC and science data interfaces use SpaceWire. BepiColombo is the first spacecraft with a network application of SpaceWire interfaces. The MPO provides all the intelligence during cruise and is enhanced with additional data buses to the MMO and MTM for this purpose.

Permanent availability of a functioning processor to guarantee safe and prompt attitude control is provided in Survival Mode by redundant FCEs (Failure Control Electronics) which take over the control functions in the event of an OBC reboot. The FCEs retain control for 7 minutes after which it is taken over by the reconfigured OBC.

The SSMM (Solid State Mass Memory) is a stand-alone unit in the BepiColombo MPO DMS (Data Management System). The SSMM interfaces with the BepiColombo payload instruments and the OBC (On-Board Computer) via main and redundant SpaceWire links. The SSMM stores telemetry packets according to the CCSDS for later downlink via X- or Ka-band. The SSMM also routes telecommand (TC) packets from the OBC to the relevant payload instrument and the returning telemetry reports from instruments to OBC (Ref. 34).

SSMM has a capacity of 384 Gbit. This storage area is organized in packet stores (maximum 50 packet stores active in parallel) for telemetry data storage. There are two types of packet stores that can be created in the SSMM: cyclic packet stores - when the packet store is full, old data is overwritten; and non-cyclic packet store - when the packet store is full the data storage is interrupted (that means new data can not be stored and is lost) and an action from ground is necessary in order to free space by deleting old data via telecommand.

The telemetry science data packets are stored in the SSMM packet stores based on PIDs (Process ID). One PID can only be associated to one SSMM packet store at a time, but several PIDs can be routed to the same SSMM packet store. The instruments will generate low- and/or high-priority science data and store it in different packet stores based on the PIDs.

CPS (Chemical Propulsion Systems): The MPO CPS is tasked with the 15 MOI manoeuvres and attitude control, for which it is equipped with redundant 4 x 22 N and redundant 4 x 5 N thrusters. The 22 N thrusters are bipropellant while the 5 N thrusters are monopropellant: these are combined into the first dual-mode propulsion system implemented on a European spacecraft. The system uses hydrazine and MON (Mixed Oxides of Nitrogen). 669 kg of propellant are carried, giving a capability of 1000 m/s ΔV plus attitude control.

 

RF communications: The MPO is equipped with two fixed LGAs (Low Gain Antennas), a 2-axis steerable MGA (Medium Gain Antenna) and a 2-axis steerable 1.1 m diameter HGA (High Gain Antenna). The two X-band LGAs will provide omnidirectional coverage at small distances from Earth and can also be used for emergency commanding at any distance. The X-band MGA will be used primarily during the interplanetary cruise phase and in safe and survival modes. The HGA will provide X-band uplink and downlink and Ka-band downlink communications for spacecraft and science operations. The HGA will also be used during the cruise phase to enhance communications and data dump capabilities whenever needed. The X-band horn MGA is steerable around the MPO or MCS obstructions in order to view Earth and is the primary antenna during cruise.

The newly developed DST (Deep Space Transponder) supports telecommanding uplink in X-band with telemetry downlink in both X- and Ka-bands to enable the downlink of 1550 Gb/year of science data. The DST supports ranging in X/X-band and X/Ka-band while the Ka/Ka-band ranging is provided with the inclusion of the payload-provided MORE translator. This ranging strategy is related to the Radio Science Experiment and requires high stability of the HGA.

Power amplification is by TWTAs for both X- and Ka-bands. All antennas are exposed to the severe thermal environment and are based on titanium. The antenna pointing mechanisms for HGA and MGA are capable of operating at 250°C.

ESA's Cebreros 35 m ground station (Ávila, Spain) is planned to be the primary ground facility for communications during all mission phases. The ground stations at Kourou (LEOP), New Norcia (critical phases during cruise and Mercury capture), Perth (LEOP), Usuda (backup) and Uchinoura (backup) will be available for backup during critical flight phases and/or for use during special campaigns.

Ka-band Operations: To increase the scientific return without increasing the duration of the ground station contacts, the BepiColombo transponder includes a Ka-band transmitter in addition to the traditional X-band receiver/transmitter. Use of Ka-band on the downlink was technically validated on the Smart-1 mission of ESA, but this will be the first operational use on a scientific ESA mission.

The quality of a Ka-band link is strongly dependent on weather conditions at the ground station. This is addressed both in the S/C design and the operations approach:

- A space-to-ground closed-loop file transfer protocol is provided, allowing to automatically recover any lost data due to unpredictable Ka-band link variations, similar to a file transfer protocol used on the uplink for BepiColombo (as well as for previous ESA interplanetary missions).

- Selection of an adequate downlink rate for Ka-band operations depending on the expected weather conditions affects the scientific return: if the planning is too conservative, the advantages of Ka-band may not be fully exploited. If it is too optimistic, too much data will need to be retransmitted. While the precise operational concept is yet to be detailed (this will only be relevant for routine operations at Mercury, i.e. not before early 2025), ESOC is currently running studies dedicated to special tools that incorporate local weather forecasts for optimizing the downlink bit rate. Outcome of a first study activity was that by introducing a 1-day weather forecast in the operations concept, a potential advantage of up to 20% in data volume, could be achieved as compared to methods based on availability of seasonal or monthly statistics of the attenuation and brightness temperature.

 

Development status:

• July 6, 2017: ESA's Mercury spacecraft has passed its final test in launch configuration, the last time it will be stacked like this before being reassembled at the launch site next year. 23)

- BepiColombo's two orbiters, Japan's MMO (Mercury Magnetospheric Orbiter) and ESA's MPO (Mercury Planetary Orbiter), will be carried together by the MTM (Mercury Transport Module). The carrier will use a combination of electric propulsion and multiple gravity-assists at Earth, Venus and Mercury to complete the 7.2 year journey to the Solar System's mysterious innermost planet.

- Once at Mercury, the orbiters will separate and move into their own orbits to make complementary measurements of Mercury's interior, surface, exosphere and magnetosphere. The information will tell us more about the origin and evolution of a planet close to its parent star, providing a better understanding of the overall evolution of our own Solar System.

- To prepare for the harsh conditions close to the Sun, the spacecraft have undergone extensive testing both as separate units, and in the 6 m-high launch and cruise configuration.

- One set of tests carried out earlier this year at ESA's technical center in the Netherlands focused on deploying the solar wings, and the mechanisms that lock each panel in place. The 7.5 m-long array of the Mercury Planetary Orbiter and the two 12 m-long array of the Mercury Transport Module will be folded while inside the Ariane 5 rocket.

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Figure 12: The complete BepiColombo spacecraft stack on 5 July 2017: From bottom to top: the Mercury Transfer Module (sitting on top of a cone-shaped adapter, and with one folded solar array visible to the right); the Mercury Planetary Orbiter (with the folded solar array seen towards the left, with red protective cover), and the MMO (Mercury Magnetospheric Orbiter). The Mercury Magnetospheric Orbiter's Sunshield and Interface Structure (MOSIF) that will protect the MMO during the cruise to Mercury is sitting on the floor to the right (image credit: ESA–C. Carreau , CC BY-SA 3.0 IGO)

• June 14, 2017: Media representatives are invited to a briefing on BepiColombo, ESA and JAXA's joint mission to Mercury, and to view the spacecraft before it leaves for Europe's Spaceport in Kourou, French Guiana, for launch next year. 24)

- Mercury is the least explored planet of the inner Solar System. BepiColombo is set to follow up on many of the intriguing results of NASA's Messenger mission, probing deeper into Mercury's mysteries than ever before. It will examine the peculiarities of its internal structure and magnetic field generation, and how it interacts with the Sun and solar wind. It will investigate surface features and chemistry, such as the ice in permanently shadowed craters at the poles. The mission's science will help revolutionize our understanding of the formation of our Solar System, and in the evolution of planets close to their parent stars.

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Figure 13: The full BepiColombo stack seen in the Large European Acoustic Facility (LEAF) at ESA/ESTEC in June 2017. The walls of the chamber are fitted with powerful speakers that reproduce the noise during launch (image credit: ESA–C. Carreau, CC BY-SA 3.0 IGO) 25)

• March 6, 2017: The BepiColombo mission to Mercury is undergoing final testing at ESA/ESTEC in the Netherlands prior to its launch from Europe's Spaceport in Kourou, French Guiana in October 2018 (Figure 14). 26)

- The opening will be repeated after the spacecraft has been vibrated to simulate the conditions of launch, and again after it arrives at the launch site. - The wing will be folded against the body inside the Ariane 5 launch vehicle and will only open once in space.

- The MPO will be attached to Japan's MMO (Mercury Magnetospheric Orbiter), which will sit inside a protective sunshield. The two scientific spacecraft will be carried to the innermost planet by the MTM (Mercury Transport Module), using a combination of electric propulsion and multiple gravity-assists at Earth, Venus and Mercury.

- After the 7.2 year journey, the two will separate and make complementary measurements of Mercury's interior, surface, exosphere and magnetosphere. The data will tell us more about the origin and evolution of a planet located close to its parent star, providing a better understanding of the overall evolution of our own Solar System as well as exoplanet systems.

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Figure 14: ESA's MPO (Mercury Planetary Orbiter) first saw the 7.5 m long three-panel solar wing being attached, and then unfurled. This was the first time the array had been deployed while attached to the orbiter. The panels were held from above to simulate the weightlessness of space (image credit: ESA)

Legend to Figure 14: In this view, the solar wing is partially unfolded. The ‘back' of the wing is facing the viewer, showing the cabling that will be connected to the main body, while the reflective Sun-facing side of the panels are not seen. One of the back panels is also reflective, to deflect stray light coming from the body.

• February 21, 2017: The French space agency CNES and Roscosmos, the Russian federal space agency, have signed an agreement concerning Russia's contribution to the PHEBUS ultraviolet spectrometer designed to study Mercury's exosphere as part of the science payload on the MPO (Mercury Planetary Orbiter) for the BepiColombo mission. 27)

- CNES, which is overseeing France's contribution to the BepiColombo mission, is leading development and system-level integration of the PHEBUS (Probing of Hermean Exosphere by Ultraviolet Spectroscopy) instrument and will support French science activities throughout the operational phase of the mission. Roscosmos is developing the spectrometer scanning system for CNES and is involved in interface work on the instrument and associated testing.

- The Russian contribution is being led by IKI RAN, the Space Research Institute of the Russian Academy of Sciences, mandated by Roscosmos. In France, the LATMOS atmospheres, environments and space observations laboratory, part of the CNRS national scientific research center, has been selected for this mission.

• January 26, 2017: BepiColombo, Europe's first mission to Mercury, is currently being put through its paces at ESA/ESTEC (European Space Research and Technology Center) in the Netherlands. Mechanical and vibration tests will get underway in April with a view to a launch in October 2018. BepiColombo will arrive at Mercury, the smallest planet in our Solar System, in December 2025. 28)

- The ESA-led joint European and Japanese mission consists of two spacecraft -MPO (Mercury Planetary Orbiter) and MMO (Mercury Magnetospheric Orbiter) - as well as a sunshield and a Mercury Transfer Module, which will power its seven year journey using its solar electric propulsion engine. It will be a mission of further discovery after NASA's Messenger spacecraft uncovered a number of surprises - including evidence of water ice at the closest planet to the Sun and a magnetic dipole field.

• September 6, 2016: The MTM (Mercury Transfer Module) will carry Europe's MPO (Mercury Planetary Orbiter) and Japan's MMO (Mercury Magnetospheric Orbiter) together to the Sun's innermost planet. The four ion thrusters are positioned at the bottom of the spacecraft (Figure 15), known as the ‘engine bay', which provides the thrust during the mission's journey, including long firing periods lasting several months at a time. 29)

- "Completing the integration of the solar electric propulsion thruster floor is a major achievement for the BepiColombo project," says project manager Ulrich Reininghaus.

- By ionizing their propellant plume using electrical energy from the solar panels, the T6 thrusters can accelerate BepiColombo with an efficiency 15 times greater than a conventional chemical thruster.

- The work took place at ESA/ESTEC in the Netherlands, the largest spacecraft testing facility in Europe. The 22 cm diameter T6 was designed for ESA by QinetiQ in the UK, whose expertise in electric propulsion stretches back to the 1960s.

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Figure 15: The base of the MTM (Mercury Transfer Module) with its four T6 ion thrusters is fully fitted for its 6.5 year journey to Mercury, along with the rest of the BepiColombo spacecraft (image credit: ESA)

• April 27, 2016: A quartet of highly efficient T6 thrusters is being installed on ESA's BepiColombo spacecraft to Mercury at ESA's ESTEC Test Center in Noordwijk, the Netherlands (Figure 16). The 22 cm diameter T6 was designed for ESA by QinetiQ in the UK, whose expertise in electric propulsion stretches back to the 1960s. It is an scaled-up version of the 10 cm T5 gridded ion thruster, which played a crucial role in ESA's GOCE gravity-mapping mission by continuously compensating for vestigial atmospheric drag along its extremely-low orbit. 30)

- The Mercury Transfer Module will carry Europe's Mercury Planetary Orbiter and Japan's Mercury Magnetospheric Orbiter together to Sun's innermost planet over the course of 6.5 years. "BepiColombo would not be possible in its current form without these T6 thrusters," explains ESA propulsion engineer Neil Wallace. "Standard chemical thrusters face a fundamental upper limit on performance, set by the amount of energy in the chemical reaction that heats the ejected propellant producing the thrust. "Ion thrusters can reach much higher exhaust speeds, typically an order of magnitude greater, because the propellant is first ionized and then accelerated using electrical energy generated by the solar panels. The higher velocity means less propellant is required.

- "The down side is that the thrust levels are much lower and therefore the spacecraft acceleration is also low – meaning the thrusters have to be operating for long periods. However, in space there is nothing to slow us down, so over prolonged periods of thrusting the craft's velocity is increased dramatically. Assuming the same mass of propellant, the T6 thrusters can accelerate BepiColombo to a speed 15 times greater than a conventional chemical thruster."

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Figure 16: The eerie blue exhaust trail of a T6 ion thruster, a quartet of which will transport BepiColombo towards the innermost planet (image credit: NASA/JPL)

• January 27, 2016: The BepiColombo orbiter completed compatibility testing at ESTEC. The spacecraft was subjected to two tests by the project team: 31)

- First, the craft was checked for electrical compatibility with the electrical field generated by the Ariane-5 launcher that will deliver it into orbit, with no possibility of interference with BepiColombo's receivers.

- Secondly, incompatibility tests between the different subsystems of the spacecraft itself were performed when it orbits Mercury. In particular, the team checked that its trio of antennas on top can communicate properly with Earth.

- The orbiter was positioned to allow deployment of its medium-gain antenna in terrestrial gravity. The high-gain antenna reflector meanwhile was deployed in a worst-case position, supported by a dedicated fixture. The spacecraft was tilted by means of a large platform while the high-gain antenna was supported by a tower made of wood, transparent to radio waves. All test cables used were shielded to reduce potential interference.

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Figure 17: The orbiter underwent ‘electromagnetic compatibility, radiated emission and susceptibility' testing last month inside the Maxwell chamber of ESA's ESTEC Test Center in Noordwijk, the Netherlands (image credit: ESA, G. Porter, CC BY-SA 3.0 IGO)

• July 22, 2015: The antenna that will connect Europe's BepiColombo with Earth is being tested for the extreme conditions it must endure orbiting Mercury. The trial is taking place over 10 days inside the LSS (Large Space Simulator) at ESA/ESTEC. The 1.5 m diameterHGA (High Gain Antenna), plus its boom and support structure, are subjected to a shaft of intense sunlight in vacuum conditions, while gradually rotated through 90º. 32)

- The mammoth chamber's high-performance pumps create a vacuum a billion times lower than standard sea-level atmosphere, while the chamber's black interior walls are lined with tubes pumped full of –190°C liquid nitrogen to mimic the extreme cold of deep space. At the same time, the hexagonal mirrors seen at the top of Figure 18, reflect simulated sunlight onto the satellite from a set of 25 kW bulbs more typically employed to project IMAX movies. In this case, the alignment of the 121 mirrors was adjusted to tighten the focus of their light beam, reproducing the intensity of sunlight experienced in Mercury orbit – around 10 times more intense than terrestrial illumination.

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Figure 18: Photo of the BepiColombo antenna in LSS (image credit: ESA)

• May 15, 2015: The MIXS (Mercury Imaging X-ray Spectrometer), a UK-built instrument, has been shipped from the University of Leicester's Space Research Centre to the European Space Agency where it will be integrated with the BepiColombo spacecraft. 33)

• Fall 2014: Preparation of the MPO (Mercury Planetary Orbiter) FM spacecraft for the thermal system test continued as planned. The test configuration is complete and a final dress rehearsal test was conducted. Installation of the outer skin progressed on the spacecraft; loading in the test chamber occurred on 24 October. 34)

- The MTM (Mercury Transfer Module) had also been delivered to ESTEC in July 2014. The thruster floor with pre-integrated thruster pointing mechanisms was delivered and verified with the high-pressure regulator and electronics unit.

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Figure 19: Photo of the BepiColombo spacecraft as it is moved into ESA's space simulator (image credit: ESA) 35)

• January 2014: The BepiColombo CDR (Critical Design Review) was completed. The MPO (Mercury Planetary Orbiter) PFM (Proto-Flight Model) AIT (Assembly, Integration and Test) continued at Thales Alenia Space Italy in Turin with Integrated System Tests (ISTs) on individual subsystems. ISTs for the DMS (Data Management Subsystem) and the Communication Subsystem were completed. Three FM and two QM payload instruments (MERMAG, MERTIS, MGNS, PHEBUS QM and BELA QM) were integrated and functionally verified on the MPO. The SERENA instrument was delivered, while electrical integration of the PCDU FM (Flight Model) is in progress. 36)

- Most of the spacecraft harness was integrated on the MTM (Mercury Transfer Module) PFM in Turin. Anomalies that occurred in MTM solar panel substrate manufacturing are being fixed with a dedicated sample test program. MPO panel manufacturing is on hold, pending agreement on whether additional process modifications are necessary in addition to the general improvements in process control identified for MTM panel production.

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Figure 20: Illustration of the BepiColombo MCS (Mercury Composite Spacecraft), image credit: Airbus DS (Ref.21)


Launch: BepiColombo is planned to be launched in October 2018 by Ariane-5 from Europe's Spaceport in French Guiana. 37) 38) 39)

The launch delay decision was made after a major electrical problem was detected during preparations for a thermal test of the MTM (Mercury Transfer Module), one of the major spacecraft elements of BepiColombo. The six-month postponement will have no impact on the science return of the mission. However, the new flight time to Mercury will be 7.2 years, and BepiColombo will now arrive in December 2025, one year later than previously anticipated. The seven-year cruise to the innermost planet of our Solar System will include 9 flybys of Earth, Venus and Mercury. The MTM, MPO and MMO are currently undergoing intensive tests in ESA/ESTEC (European Space Research and Technology Center) in the Netherlands. Everything is going well with the MPO and MMO. The last of the instrument flight models was installed recently on the MPO (Ref. 37).

Orbit: The launch will be followed by a 7.2 years cruise phase, including planetary swingbys at Venus and Mercury, eventually achieving a weak capture by Mercury in December 2025 (1 year later than previously planned). During the cruise phase, electric propulsion will be used for extended periods of time. This is provided by the MTM module, which will be jettisoned at Mercury arrival. The seven-year cruise to the innermost planet of our Solar System will include 9 flybys of Earth, Venus and Mercury. — A set of complex maneuvers will deliver the MMO to its operational orbit, and finally the MPO will be put into a 1500 x 480 km polar orbit (orbital period of about 2.2 hr) to start its scientific mission, planned to last for one Earth year (with a 1 year extension capability).

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Figure 21: Cruise trajectory for April 2018 launch, showing the sun distance, SEP (Solar Electric Propulsion) usage, and planetary flybys (image credit: ESA)

Figure 22 shows the BepiColombo spacecraft. The combined stack can have the following configurations:

1) MCSC (Mercury Composite Spacecraft - Cruise): MTM, MPO, MMO sunshield (MOSIF) and MMO

2) MCSA (Mercury Composite Spacecraft - Approach): MPO, MOSIF and MMO following separation of the MTM

3) MCSO (Mercury Composite Spacecraft - Orbit): MPO and MOSIF following release of the MMO.

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Figure 22: Artist's impression of BepiColombo in cruise configuration (top), with the various elements of the cruise stack in exploded view (bottom left), and the MPO at Mercury (bottom right), image credit: ESA

MAP (Mercury Approach Phase): The MAP starts after the last electric propulsion maneuver has been completed, approximately two months before the first Mercury orbit insertion maneuver. During this phase, the MTM will separate from the spacecraft stack. The remaining composite of MPO/MMO/MOSIF, the MCSA configuration, will drift into Mercury's sphere of influence, and will need only a small maneuver to get captured in an initial orbit of approximately 590 x 178,000 km ((April 2018 launch scenario). This process is known as a 'weak stability boundary' capture.

MOI (Mercury Orbit Insertion) phase: The MOI starts thereafter, including a series of chemical propulsion maneuvers with the aim of achieving the operational orbit firstly for the MMO (11,639 x 590 km, i=90º, RAAN=67.8º, ω=-2º) and eventually for the MPO (1500 x 480 km, i=90º, RAAN=67.8º, ω=16º).

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Figure 23: Schematic of the MOI sequence for launch in April 2018 (depicted in ecliptic J2000 frame), image credit: ESA

Operations in the MOI phase are driven by the following main constraints:

- Below a certain altitude, the S/C rotation around the sun line has to be synchronized with the orbital motion around Mercury, to ensure thermal limits are not violated.

- Maneuvers shall not take place around Mercury perihelion ±60 deg due to thermal constraints.

- The S/C undergoes eclipse seasons during MOI, which are power-critical in the higher orbits. Special operational measures like boost heating prior to eclipse entry are expected to be required for ensuring a positive power budget. It is imperative to sufficiently lower the orbit prior to the aphelion eclipse season. In particular, a failure to separate the MMO as planned prior to the eclipse season could be mission-critical in case the orbit has not been lowered sufficiently.

- Operational constraints on maneuver execution: a delta time of at least 3 days is observed between maneuvers. No maneuvers are allowed during solar conjunction periods (no ground contact possible) and as from 7 days before (a failed maneuver shortly before a solar conjunction may lead to the S/C using incorrect guidance and hence a violation of thermal constraints, with no ground intervention possible).

This leads to a rather constrained MOI timeline as shown in Figure 23. Five initial burns are performed to reduce the apoherm altitude to the MMO target value of 11,639 km. Following separation of the MMO, the MOSIF is separated shortly after, bringing the S/C into MPO configuration. Another 10 maneuvers are then required to achieve the MPO operational orbit. Duration of the MOI phase is about 3 months, with a total ΔV for the sequence shown in the figure of about 963 m/s.

Mercury Orbit Phase: Once the MPO mission orbit is reached, the final commissioning of the MPO and its payload is performed; this will last about one month. The MPO attitude follows a continuous nadir-pointing profile, providing optimum viewing conditions for the payload.

All MPO science data will be stored in the spacecraft's solid-state mass memory and downlinked during daily station passes with ESA's Cebreros ground station. Every half Mercury year, about every 44 Earth days, the attitude of the spacecraft will have to be reversed around the nadir direction to keep the radiator pointing away from the Sun.

The MMO will communicate with the JAXA/ISAS Sagamihara Space Operations Center via the Usuda Deep Space Center (UDSC) 64 m antenna in Nagano, Japan.

Nominal mission science operations are scheduled to be performed for one Earth year, with a planned extension of another year.

 


 

MPO sensor complement: (BELA, ISA, MERMAG, MERTIS, MGNS, MIXS, MORE, PHEBUS, SERENA, SYMBIO-SYS), SIXS)

MPO will carry a highly sophisticated suite of eleven scientific instruments, ten of which will be provided by Principal Investigators through national funding by ESA Member States and one from Russia (Ref. 3). 40)

BELA (BepiColombo Laser Altimeter)

BELA will characterize the topography and surface morphology of Mercury. It will also provide a digital terrain model that, compared with the data from the MORE instrument, will give information about the internal structure, the geology, the tectonics and the age of the planet's surface. Co-PIs (Principal Investigators): Nicolas Thomas, University of Bern, Switzerland, and Tilman Spohn, DLR, Germany. Further partners are the MPS (Max Planck Institute of Solar System Research) and the IAA (Instituto de Astrofisica de Andalucia). 41) 42)

BELA is the first European laser altimeter to be built for inter-planetary flight. A key element has been the development of a European high-power (50 mJ) pulsed Nd:YAG laser allowing instrument operation at distances of > 1055 km from the target. The nadirpointing geometry of MPO necessitated the use of baffles to reject the incoming sunlight (when MPO is over the nightside hemisphere but still illuminated by the Sun). The strict mass constraints combined with the expected large temperature excursions (arising in large part from the eccentricity of Mercury's orbit) drove the selection of a beryllium telescope as receiver. In addition, the competences within the participating countries led to adoption of a digital rangefinder concept.

Laser: The laser is a fully redundant 1064 nm Nd:YAG with 5 ns pulse duration and a nominal 50 mJ pulse energy. A beam expander collimates the beam to a 60 µrad width. The system can operate at up to 10 Hz, consumes 20 W with a mass of < 5 kg (including MLI, cabling, beam expander, and drive electronics).

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Figure 24: Schematic view of the BELA instrument and its components ( BELA team)

Baffles: The receiver baffle follows a Stavroudis concept. It is an aluminium structure combining ellipsoidal and hyperbolic surface machined with 4 nm roughness. The internal diameter is 204 mm and an extremely thin wall thickness has been achieved to minimize mass. Although the transmitter baffle is smaller, it must also hold a thermal filter to prevent the beam expander focussing reflected light from Mercury on to the laser. A narrow band transmits the laser wavelength but rejects light outside a band around this wavelength.

Telescope: The receiver telescope is a two-mirror on-axis design with a 20 cm primary. The telescope is an all-beryllium design with a mass of roughly 600 g. The primary mirror is just 2 mm thick. The telescope surfaces have been produced using diamond-turning of a deposited copper layer followed by gold coating. The aperture at the vertex of the primary mirror is close to the focus of the telescope and supports the instrument straylight rejection concept.

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Figure 25: The receiver telescope which is being manufactured out of beryllium (image credit: BELA team)

Rangefinder module: Unlike previous planetary laser altimeters, the rangefinding of BELA is performed using a digital approach where the signal is digitized and the return pulse detected using software in an FPGA. The resolution is limited by the digitization frequency and the bandwidth but tests indicate that in optimum conditions, accuracies of the order of 20 cm over the (typically) 500 km range can be achieved. The rangefinder can also detect fairly low return pulse energies. Testing indicates that a return pulse containing just 6 photons can be detected. The final system will have inferior performance because of noise contributions and the effects of radiation damage in flight. However, from a technological standpoint, the ranging system will meet the requirements foreseen in the original BELA proposal.

BELA operations: The 60 µrad wide beam is reflected from the surface (surface spot size = 20-50 m) and received around 5 ms later at a 20 cm diameter f/5 telescope. The image is refocussed onto a silicon avalanche photodiode through a narrow bandpass interference filter. The signal is then sampled and fed to a digital pulse discrimination electronics. This system determines the time of flight (and therefore range), the integrated pulse intensity, and its width. The data are passed to a digital processing unit which controls the operation and services the spacecraft interface. Onboard data compression and data storage are foreseen. The experiment requires significant baffling and thermal control but can operate over the dayside hemisphere (with only slightly reduced signal to noise) allowing optimum data acquisition over a minimum duration. BELA will provide 2 ns time resolution (30 cm range) which is commensurate with the expected knowledge of the spacecraft position. Optimum data return is expected at altitudes up to at least 1000 km above the surface. Samples will be acquired about every 250 m on ground-tracks separated by 25 km at the equator (crossing at the poles). Over the lifetime of the mission, data points will be 6 km apart (decreasing with latitude). The experiment will provide return pulse intensity and width information allowing an assessment of surface albedo and roughness at 20 m scales including in unilluminated polar craters.

 

ISA (Italian Spring Accelerometer)

The objectives of the ISA accelerometer are strongly connected with those of the MORE experiment. Together the experiments can give information on Mercury's interior structure as well as test Einstein's theory of the General Relativity. PI: V. Iafolla, CNR-IFSI, Italy. 43) 44) 45)

ISA is a three–axis high-sensitivity accelerometer, characterized by an intrinsic noise level of about 10-10g/√Hz in the frequency band 3 x 10-5 — 1 x 10-1 Hz. The main goal of ISA it to measure the very strong non–gravitational accelerations acting on the MPO (Mercury Planetary Orbiter) spacecraft, which are an important source of error in the RSE (Radio Science Experiments) measurements. The non-gravitational accelerations are proportional to the area-to-mass ratio of the spacecraft, and are very difficult to be properly modeled for a complex in shape and active satellite like the MPO. 46)

The main objectives of RSE are to perform precise measurements of:

• gravitational field of Mercury

• rotation of Mercury

• general relativistic effects, in particular Mercury perihelion precession by state-of-the-art radiometric tracking of the MPO spacecraft.

Indeed, the modeling depends on a set of parameters related with the physical properties of the satellite surface and structure, which will be strongly influenced, and with completely unknown laws, by the strong radiation environment in the surroundings of Mercury. In order to reach the ambitious objectives of the RSE, the a posteriori reconstruction of the MPO orbit should reach the 10-8 m/s2 level in acceleration over a time span of one orbital revolution of the spacecraft around Mercury, i.e. about 2.3 hours. The ISA measurements have to be integrated with the radar tracking measurements from Earth's stations in a very precise orbit determination procedure. The RSE are a complex mix of measurements and scientific objectives, and it is not possible to separate them neatly in independent experiments. These experiments are based, from one side, on a sophisticated and very precise tracking system, both in range and range-rate, that will use a full 5-way frequency link from Earth's ground stations to the MPO (X-band, Ka-band and a mixed mode).

From the other side, a precise orbit determination software and procedure is needed in order to reconstruct the orbit of the MPO around Mercury, and of Mercury center-of-mass around the Sun, while solving in a complex least-squares fit for local and global parameters. Finally, the ISA measurements will also be useful to estimate the speed variations produced by the onboard thrusters firings during the offloading maneuvers of the spacecraft reaction wheels, at least once every 24 hours.

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Figure 26: Illustration of the ISA elements (image credit: CNR-IFSI)

 

MERMAG (Mercury Magnetometer)

The MPO's MERMAG will provide measurements that will lead to the detailed description of Mercury's planetary magnetic field and its source, to better understand the origin, evolution and current state of the planetary interior, as well as the interaction between Mercury's magnetosphere with the planet itself and with the solar wind. PI: Karl-Heinz Glassmeier, Technical University of Braunschweig, Germany Co-PI: C.M. Carr, Imperial College London, UK.

MERMAG consists of magnetometers on board MPO and MMO: MPO-MAG and MMO-MGF. Some measurements are only possible using the magnetometers on both spacecraft. MPO-MAG is a dual digital fluxgate magnetometer, which shall be used to measure DC and low frequency perturbations of the magnetic field. 47)

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Figure 27: Schematic of Mercury's magnetic field (image credit: MERMAG team)

The primary objective of the magnetic field investigation on MPO is to provide the magnetic field measurements that will lead to the detailed description of Mercury's planetary magnetic field, and thereby constrain models of the evolution and current state of the planetary interior. This objective will be achieved using accurate magnetic field measurements by MERMAG on MPO. It will be supported by measurements made on the MMO (Mercury Magnetospheric Orbiter), both to distinguish the effects of the Hermean magnetosphere on the MPO measurements and to use the MMO measurements directly to augment the database for the determination of the internal terms. With the data of MERMAG, it will also be possible to determine all the terms associated with the internal field up to the octopole with high accuracy as well as higher order terms, depending on the structure of the internal field.

The secondary objectives of MERMAG relate to the interaction of the solar wind with the Hermean magnetic field and the planet itself, the formation and dynamics of the magnetosphere as well as to the processes that control the interaction of the magnetosphere with the planet. In particular, measurements close to the planet will allow the determination of the conditions for access of the solar wind to the planetary surface and assessing the role and importance of different current systems, including subsurface induction currents and the conductivity of the regolith. These objectives will be greatly assisted by the planned close association with the magnetic field investigation on the MMO.

Measurement principle: MPO-MAG is designed as follows: Two identical magnetometers are used each with their own dedicated electronics. This two sensors technique will be applied in order to help determine the magnetic influence of the spacecraft. The instrument hardware comprises an electronics box, two sensor units with their associated thermal hardware and mechanical fixings, plus an electrical harness which connects the sensors to the electronics box. The sensors are mounted on a deployable boom, whilst the electronics box is located inside the spacecraft structure.

The boom is a critical subsystem both for the MPO-MAG instrument and the spacecraft. It enables both sensors to be slightly removed from the spacecraft; combining the signal from both the inboard and outboard sensors will help determine the magnetic interference from the spacecraft itself.

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Figure 28: Schematic of boom mounting technique (image credit: MERMAG team)

The MPO-MAG instrument is largely autonomous in operation, requiring a minimum of commanding only for selecting from a set of science operations modes and corresponding telemetry bit rates. The two sensors measure the magnetic field with a sample rate of 128 Hz. These data will be reduced onboard to a lower temporal resolution depending on the instrument mode: 64, 32, 16, 8, 4, 2, 1, and 0.5 Hz.

 

MERTIS (Mercury Radiometer and Thermal Infrared Spectrometer)

The objective is to provide high spectral resolution data. MERTIS will return detailed information about the mineralogical composition of Mercury's surface layer. This is crucial for selecting a valid model for the origin and evolution of the planet. PI: Harald Hiesinger, University of Münster, Co-PI: Jorn Helbert, DLR, Germany. MERTIS is a joint project of the University of Münster, two institutes of the German Aerospace Center (DLR) in Berlin-Adlershof and several industrial partners and research institutes. 48) 49) 50)

MERTIS has four scientific goals: the study of Mercury's surface composition, identification of rock-forming minerals, mapping of the surface mineralogy, and the study of the surface temperature variations and thermal inertia. The instrument will provide detailed information about the mineralogical composition of Mercury's surface layer by measuring the spectral emittance in the spectral from 7-14 µm with a high spatial and spectral resolution. Furthermore MERTIS will obtain radiometric measurements in the spectral range from 7-40 µm to study the thermo-physical properties of the surface.

MERTIS is a pushbroom radiometer. The spectrometer employs an uncooled microbolometer array made from amorphous silicon, which yields a short thermal time constant as well as very low NETD (Noise Equivalent Temperature Difference). The array provides spectral separation and spatial resolution according to its two-dimensional shape. The operation concept principle is characterized by intermediate scanning of the planet surface and three different calibration targets: free space and on-board black body sources.

The general instrument architecture comprises two separate parts - the Sensor Head including optics, detector and proximity electronics and the Electronics Unit containing the power supply with an interface to the primary bus, the sensor control and the driving electronics. This highly integrated and low mass (3.3 kg) measurement system is completed by a motor driven Pointing Unit device which orients the optical path to the planet and the calibration targets.

 

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Figure 29: Illustration of the MERTIS instrument configuration and its elements (image credit: MERTIS team)

On Mercury, the spectral radiance at day side shows that the thermal emission starts to dominate the radiance already at wavelengths larger than 1.2 µm (at 725 K) depending on the surface albedo. The range between 0.8 and 2.8 µm is a transition region characterized by the overlapping of the reflected solar radiation and the thermal emission. However, Mercury's thermal flux exceeds the flux reflected from its surface. This enables emittance spectroscopy in the thermal IR range where there is high potential for mineral identification because it is in this region where the major rock-forming minerals (e.g. feldspar) have their fundamental vibration bands.

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Figure 30: MERTIS instrument overview (image credit: MERTIS team)

The optics design is is based on an all-reflective optics concept with an off-axis TMA (Three Mirror Anastigmatic) telescope behind an IR entrance window and a pointing mirror for the target selection (planet view or calibration views to deep space and two reference sources). The spectrometer is a derivative of an elegant relay disclosed by Offner in the early 1970s. This combination is free from spherical aberration, coma and distortion and, when the algebraic sum of the powers of the mirror reflecting surfaces utilized is zero, the image produced is free from third order astigmatism and field curvature.

Parameter

Spectrometer

Radiometer (µRAD)

Focal length

50 mm

F number

2.0

Microbolometer array
Detector
Illuminated pixels

160 x 120 @ 35 µm
100 spatial, 80 spectral
2 x 15 @ 250 µm

Spectral channel width

90 nm / pixel

Spectral resolution

78 – 156 λ/Δλ

Spectral range

7 – 14 µm

7-40 µm

Detectivity

0.95 x109 cm √Hz W-1

7 x 108 cm √Hz W-1

IFOV (Instantaneous Field of View)

0.7 mrad

5 mrad

GSD (Ground Sample Distance)
- Periherm 400 km
- Apoherm 1500 km


280 - 1400 m (M= 1- 5)
1050 m


2000 m
7500 m

FOV (Field of View)

4º ACT (Across Track), 0º ALT (Along Track)

4º ACT, 1º ALT

Swath width

28 km

Table 4: Key optics parameters of MERTIS

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Figure 31: Optics configuration of MERTIS (image credit: DLR)

Radiometer: The basic concept of the radiometer channel is to place a 2 x 15 elements thermopile double line array sensor chip with integrated optical slit for the spectrometer at the focal plane of the TMA entrance optics. The small signal voltages of the order of µV to mV generated by the thermopile sensors are transmitted differentially via a starr-flex interface PCB (Printed Circuit Board) to a proximity electronics, where the signals are multiplexed, amplified, converted to digital units and transmitted to the MERTIS ICU (interface Control Unit).

The unusual solution of incorporating the optical slit into the radiometer chip was driven by the very small space available which is mainly a consequence of the MERTIS requirement to minimize the heat input through the entrance optics. This design requires a modification of the standard thermopile design where a self-supporting membrane containing the thermoelectrically active layers is spanned over a surrounding Si frame. Here, an additional central bridge is added to the Si frame which provides additional mechanical support (Figure 32), thereby allowing to cut the slit into the center of the membrane. Furthermore, the thermopile pixels are only weakly coupled to this central Si bridge by very narrow V-shaped bars to reach the maximum possible sensitivity.

Each thermopile pixel of 200 µm x 1100 µm size consists of 14 thermocouples connected in series using Bi0.87Sb0.13/Sb as thermoelectric materials. The pixels are coated by a thin layer of black silver smoke, a material with nearly constant high absorption from the visible to the far infrared. The pixel width of 200 µm is close to the diffraction limit of the optics at 40 µm wavelength, a small gap of 50 µm width between the pixels is necessary for technological reasons and also effectively eliminates thermal crosstalk between neighboring pixels (under vacuum).

One infrared 8-14 µm bandpass filter is mounted directly above the thermopile array on 50 µm high standoffs which are micro machined directly onto the chip. The other array uses only the MERTIS entrance filter to realize a 7-40 µm broadband IR channel which is aimed at measuring low object temperatures down to 100 K.

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Figure 32: µRAD detector chip and part of I/F flexboard (without IR filter), image credit: MERTIS team

The electrical interface of the thermopile arrays is provided by direct wire bonding of the sensor signals to a starr-flex PCB, the ends of the two flex wires are equipped with a Nano-connector to feed the signals into the radiometer electronics. Chip and PCB are mounted inside a dedicated Aluminum housing comprising of a baseplate and a cover. The whole detector unit is then fixed to the TMA structure by Shapal spacers which are providing an excellent thermal coupling but electrical insulation.

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Figure 33: Photo of the MERTIS flight model, the housing is ~180 x 180 x 130 mm in size, the planet baffle is ~200 mm long, space baffle on top (image credit: MERTIS team, Ref. 49)

 

MGNS (Mercury Gamma-ray and Neutron Spectrometer)

MGNS will determine the elemental compositions of the surface and subsurface of Mercury, and will identify the regional distribution of volatile depositions on the polar areas which are permanently shadowed from the Sun. PI: I. Mitrofanov, IKI (Institute for Space Research), Moscow, Russia. 51) 52)

Since Mercury lacks a thick atmosphere, its natural nuclear emissions can be detected from orbit, i.e., gamma rays arising from cosmic-ray interactions and those arising form the natural radioactive decay of K (Potassium), Th (Thorium) and U (Uranium). Mercury has a very weak magnetic field with cut-off rigidity near the equator of ~1 MeV. Therefore, the galactic cosmic rays are essentially unimpeded and interact directly with the shallow subsurface producing copious secondary neutrons within the first 1–2 m of the surface (Figure 34). These neutrons interact with the soil nuclei either by in-elastic scattering or capture reactions, producing secondary nuclear gamma rays. Each chemical element has a unique set of nuclear lines, so the data from a gamma-ray spectrometer in near-orbit can, in principle, uniquely identify the elemental composition of the Mercury shallow subsurface.

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Figure 34: Galactic cosmic rays produce secondary neutrons which induce gamma-ray line emission from the surface of Mercury. Line emission also results from natural radioactive isotopes in the surface regolith (image credit: IKI)

The intensity of a gamma-ray line of a particular element depends on the spectrum and flux of secondary neutrons and so knowledge of the spectral density of neutrons is also a necessary prerequisite for the determination of the elemental abundance. The energy spectrum of leakage neutrons, in turn, depends on the elemental composition of the soil. A neutron with a mass m, loses a small fraction of energy -m/(M+m) in a collision with heavy nucleus of mass M. However, when m=M, the incident particle will lose half its energy, as is the case when a neutron collides with a hydrogen nucleus. Thus, it can be seen that the addition of even a little hydrogen into a soil will decrease the leakage flux of epithermal and high-energy neutrons while simultaneously increasing the flux of thermal neutrons.

The objective of MGNS is to observe neutron fluxes in wide energy range (from thermal to 10 MeV) and gamma-ray with high energy resolution (approximately 3.5% at the energy of 662 keV) in the energy range from 300 keV to 10 MeV during the interplanetary cruise phase and on the orbit around Mercury.

Physical characteristics of Mercury nuclear emission

Requirements for MGNS measurements

Detectors and initial data products of MGNS instrument

Flux of gamma-ray lines from the Mercury subsurface

To measure the set of the most intense gamma-ray lines, which characterize the content of soil-composing elements and natural radio-isotopes

Scintillation detector of gamma-rays SCD/G with the high spectral resolution and high efficiency for gamma-rays Data product is energy spectrum of counts for gamma-rays with 4096 linear channels at the energy range 0.3–10.0 MeV

Flux of thermal neutrons from the Mercury surface

To measure the flux of thermal neutrons below the threshold of 0.4 eV

Detectors SD1 and SD2 with 3He proportional counters, with and without Cd shielding, respectively Data product is the time profile of counts for thermal neutrons, which is determined, as difference of counts from SD2 and SD1

Flux of epithermal neutrons from the Mercury surface

To measure the flux of epithermal neutrons in two energy ranges 0.4 eV–1 keV and 0.4 eV–500 keV

Detector SD2 with 3He proportional counter and with Cd shielding for energy range 0.4 eV–1 keV. Detector MD with 3He counter and polyethylene moderator inside Cd shield for energy range 0.4 eV–500 keV. Data products are two time profiles of counts for epithermal neutrons from SD2 and MD

Flux of high-energy neutrons from the Mercury surface

To measure the flux of high-energy neutrons in the energy range 0.3–10.0 MeV

Sthylbene scintillator SCD/N within a anti-coincidence plastic scintillator APC Data product is the energy spectrum of counts for high-energy neutrons with 16 energy channels for energy range 0.3–10.0 MeV

Table 5: Measurements, detectors and initial data products of MGNS

The MGNS has one detector (SCD/G) for gamma rays and four detectors (SD1, SD2, MD and (SCD/N) for neutrons. Additionally, the high-energy neutron detector (SCD/N) is surrounded by the APS (Anti-coincidence Plastic Scintillator), to protect the sensitive volume of SCD/N from external charged particles. All these detectors are integrated into a single module, which also contains the electronic boards for analog signal processing, HV and LV provision, data storage, logic and interface support.

MGNS instrument: The MGNS design is based on the heritage of HEND (High Energy Neutron Detector (HEND) flown on NASA's "Mars Odyssey" mission. The HEND has successfully operated for more than 7 years in space and has returned more that 3 GByte of scientific data. The concept of the MGNS design and its sensors are shown in Figures 35 (a)–(d). A schematic of the MGNS electronics is presented in Figure 36. It consists of two detection segments: the MGRS (Mercury Gamma-Ray Spectrometer) and the MNS (Mercury Neutron Spectrometer) supported by the DLS (Digital and Logic Segment), which is based on an FPGA. The overall dimensions of the MGNS correspond to 257 x 342 x 140 mm3.

All three detectors SD1, SD2 and MD [Figure 35 (b)] have identical 3He proportional counters and analog electronics. They are based on the HEND prototype elements with counter LND 2517 having a diameter of 12.7 mm, a length of 94 mm and a pressure of 6 atmospheres. The digitalization of counts allows the project to record the well-known two-peak energy spectrum from 3H and p. The energy peak at 764 keV corresponds to the full energy deposition of both particles, the low-energy peak at 191 keV corresponds to the energy of 3H only, when a proton escapes from the detection volume. If necessary, one may reject the contribution of low-amplitude noise by a commendable lower-energy threshold. The front-end read-out electronics for SD1, SD2 and MD are quite simple and identical for all three sensors.

The MGNS has two scintillation sensors, SC/N and SC/G [Figure 35 (c) and (d)]. The sensor SC/N has a stilbene scintillator. It is also based on the HEND heritage. Recoil protons have randomly distributed energies from 0 up to the total energy of the neutron, En, and produce a scintillation flash in the stilbene. The light is easily detected for energetic protons above energy of -300 keV. The low-energy cut-off of the sensor SC/N is determined by this threshold. The high-energy cut-off is governed by the decreasing cross section of the recoil reaction with increasing neutron energy. We use a cylindrical stylbene crystal of size Ø30×40 cm for detector SC/N. The efficiency curve for neutron detection by the SC/N has a maximum of about a few cm2 around 2.0 MeV.

Electrons, either external, or produced by gamma rays, also generate scintillation light in the stylbene crystal, as well as protons. However, the time profiles of the scintillation flash are quite different for electrons and protons, and a special analog board of the MNS segment separates counts into these two categories. It has a high accuracy for separating electrons and protons with a mis-identification of only 1 case in 2000. External cosmic ray protons have also to be separated from recoil protons. Similarly to the design of HEND, a plastic scintillator surrounds the stilbene crystal for the rejection of external protons. An event in the plastic is used for the generation of a veto signal for rejecting cosmic ray events in the stilbene.

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Figure 35: Schematic views of the MGNS (image credit: IKI)

The gamma-ray spectrometer of the MGRS [Figure 35 (d)] is based on a LaBr3 scintillation crystal with a size of about 8 cm (both in diameter and length). For a spectral resolution of 3% at 662 keV, one would like to have about 8 energy channels over the Gaussian profile of the spectral line. Therefore, events from this sensor are converted into an energy spectrum with 4096 linear channels over the energy range 300 keV–10.0 MeV.

The architecture of the DLS (Digital and Logic Segment) is based on a radiation-resistant Actel FPGA (Figure 36). The logic of the FPGA is developed in accordance with project requirements and specifications. Low- and high-voltage supply units are operated by the DLS with the possibility of changing the levels of HV by commands. The DLS unit also provides the interface with the spacecraft systems for power, thermal control, data readout and commanding.

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Figure 36: The main elements and units of the MGNS (image credit: IKI)

 

MIXS (Mercury Imaging X-ray Spectrometer)

MIXS will use X-ray fluorescence analysis to produce a global map of Mercury's surface atomic composition at high spatial resolution. This technique has also been used by the D-CIXS instrument on ESA's Smart-1 mission to the Moon. PI: Emma Bunce, University of Leicester SRC (Space Research Center), Leicester, UK. MIXS is funded in part by the UKSA (UK Space Agency) and will use novel X-ray optics to determine small-scale features on Mercury in order to find out what the planet's surface is made of. It will do this by measuring fluorescent X-rays that come from the planet's surface, excited by high energy X-rays from the Sun, to identify chemical elements. -The MIXS contribution in Germany is funded by the Max-Planck-Society. MPS (Max Planck Institute for Solar System Research) is responsible for the scientific calibration of the detector unit and MPE is providing the whole detector flight hardware.

The MIXS instrument is designed to determine the surface composition of the planet by means of fluorescent X-ray spectroscopy. MIXS is the essential monitor of the solar X-ray flux which, along with solar wind protons, excites the characteristic K-series lines of the elements present in surface material. MIXS is also concerned with the interaction of the solar wind with Mercury's magnetosphere and exosphere which is expected to produce intensive emission of continuum X-rays. 53) 54)

The MIXS-T channel of the instrument features the first true imaging X-ray telescope to be used in planetary science – realized using MCP (Microchannel Plate) optics manufactured by Photonis SAS (Brive, France). MIXS-T will have an angular resolution of 2-4 arcmin, sufficient to resolve surface features, dependent on solar flare state, smaller than ~20 km across. A second, conventional non-imaging collimated channel is provided by MIXS-C with superior grasp to MIXS-T, but no spatial resolution on scales less than the field-of-view. The detectors for both channels are 64 x 64 pixels in a macropixel DEPFET (Depleted P- Channel Field Effect Transistor) active pixel sensor geometry (Max Planck Semiconductor Laboratory, Munich). The energy resolution of these devices is expected to be sufficient to resolve the K series lines of all the elements of interest. An excellent low-energy detector response opens up the possibility of measuring Fe abundance from the L-alpha line at 0.7 keV.

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Figure 37: Photo of the MIXS FM (Flight Model) instrument (image credit: University of Leicester) 55)

 

MORE (Mercury Orbiter Radio science Experiment)

MORE will help to determine the gravity field of Mercury as well as the size and physical state of its core. It will provide crucial experimental constraints to models of the planet's internal structure and will test theories of gravity with unprecedented accuracy. Its data will be used in conjunction with those from BELA and ISA to achieve these goals. PI: L. Iess, University of Rome ‘La Sapienza', Italy. 56) 57)

The MORE hardware, a transponder enabling high accuracy range and range rate measurements, has been designed to determine the planet's gravity field to degree and order 25 and carry out improved tests of relativistic gravity. The measurements undertaken by MORE address three different areas:

- reconstruction of the planet's gravity field, the coefficients of the spherical harmonics expansion up to degree and order 25, and the Love number k2 (gravimetry experiment). The expected accuracies range from signal to noise ratio of 104 for degree 2, to SNR of 10 for degree 20.

- estimation of the rotational state of Mercury by means of obliquity and amplitude of the physical librations in longitude (rotation experiment). These measurements, carried out in collaboration with the camera team, provide the moments of inertia of the whole planet and its mantle.

- determination of the post-Newtonian parameters, the mass and the oblateness of the Sun, and the upper limits to the temporal variation of the gravitational constant G (relativity experiment).

The crucial onboard elements of the MORE experiments are:

- the Ka/Ka transponder (KaT), provided by ASI (Italian Space Agency)

- the TLC/TCM deep space transponder (DST)

- the 1.2 m high gain antenna (HGA)

- the high sensitivity ISA (Italian Spring Accelerometer).

In addition, the final global fit and orbit reconstruction will incorporate also laser-altimetric and optical observables provided by the onboard laser altimeter (BELA) and high-resolution camera (SIMBIO-SYS). The KaT and the DST are particularly important because they enable a multi-frequency radio link at X-band (7.2 GHz uplink/8.4 GHz downlink) and Ka-band (34/32.5 GHz), a configuration already exploited by the Cassini mission. Thanks to this configuration, range rate measurements will attain accuracies of 3 µm/s (at 1000 s integration time) at nearly all solar elongation angles. In the geometric optics limit, the use of multiple frequencies allows a complete cancellation of plasma noise, the dominant noise source in the S- and X-band radio links. A novel WBRS (Wideband Ranging System), based upon a pseudo-noise modulation scheme at 24 Mcps, will provide observables accurate to 20 cm (two-way).

The effects of non-gravitational accelerations on the spacecraft dynamics (quite large in the harsh hermean environment) will be removed to a large extent thanks to the ISA accelerometer. These instrument readouts will be sent to ground in the telemetry stream and referenced to the phase center of the high gain antenna. The orbit determination code will then use a smoothed version of the accelerometer measurements to integrate the equation of motion, effectively realizing a software version of a drag-free system.

Such a complex experimental setup, implemented for the first time in a planetary mission, will be used not only for the reconstruction of the Mercury's gravity field, but also for a precise reconstruction of the spacecraft orbit. Accuracies of 0.1-1 m in the radial position seem attainable. The position of the MPO in the hermean frame (whose origin is defined by zeroing the dipole terms in the harmonic expansion of the gravitational potential) will be used for the appropriate referencing of the laser altimetric measurements and the images from the high-resolution camera. The combination of altimetric and gravity measurements will provide the topographic heights, a crucial information to determine the structure of Mercury's crust and outer mantle.

The along- and across-track position of the spacecraft is crucial for the rotation experiment, aiming to determine the rotational state of the planet by means of optical tracking of surface landmark. The pole position and physical librations in longitude will be obtained from a precise geo-referencing of high-resolution images (5 m pixel size at pericenter). The final accuracy of this experiment rests not only upon an accurate knowledge of the spacecraft position, but also on the quality of the attitude reconstruction. The onboard star trackers and gyroscopes should allow an accuracy of 1-2 arcsec. In addition, the spacecraft design ensures a high stability of the optical alignment between the star trackers and the camera.

Although MORE will make use of laser altimetric and optical images to stabilize the global orbital solution, the crucial observable quantities are range and range rate. These quantities are generated at the ground station after establishing a two-way coherent link. The core element of the tracking system is the reference oscillator, a H-maser with a frequency stability of one part in 1015 over time scales of 1000 s. The orbital solution is obtained from the observable quantities by means of a least squares fit, where the state vector of the spacecraft and the parameters of the dynamical model are jointly derived.

Numerical simulations of the MORE show that a batch-sequential estimation is fully adequate to reach the mission requirements in orbit determination and gravity field reconstruction. The batch-sequential filter provides the advantages of a batch method (i.e. a-posteriori data processing and superior stability), while adding sequential updates of the dynamical model. In our approach, observation data are processed in three steps:

• Batch-sequential estimation (initialization of local and global parameters)

• Multi-arc estimation (improvement of the global parameters)

• Single arc estimation (trajectory reconstruction).

Position errors are consistently found below 10 m in the along and cross-track components, while much better accuracies are obtained for the radial component. The orbital reconstruction is therefore fully adequate to support the laser altimetric observations (accurate to 1 m in the nadir direction). In the remaining two components (along- and across-track) the orbit determination accuracies are significantly larger, but still compatible with the requirements of the libration experiment (2 arcsec for Mercury's librations in longitude). An improvement in the orbit determination is necessary for a measurement of physical librations below the arcsecond level.

Better results are expected if additional observations become available. In the current planning, BepiColombo's MPO will be tracked by two stations, namely ESA's 34 m antenna in Cebreros, supporting mission operations, and NASA's DSN (Deep Space Network) antenna DSS 25 in Goldstone (California) for the radio science experiment. The X-band Doppler data, acquired at the Cebreros antenna, may prove valuable for the estimation of the ΔVs associated to desaturation maneuvers, a major source of uncertainty in the orbital reconstruction.

 

PHEBUS (Probing of Hermean Exosphere by Ultraviolet Spectroscopy)

The PHEBUS spectrometer is devoted to the characterization of Mercury's exosphere composition and dynamics. It will also search for surface ice layers in permanently shadowed regions of high-latitude craters. PI: Eric Quemerais, LATMOS-IPSL, France. The PHEBUS consortium composed of three main partners: Tohoku University (Japan) will provide the detectors and the main entrance mirror, IKI (Russia) will implement the scanning system, and SA/IPSL (France) will take in charge the design, assembly/ test/ integration, and will also provide three small detectors (zero order monitor, Ca- and K-channels). Finally, a small optional spectro-imager (0.3 kg), implemented under the responsibility of Southwest Research Institute (USA), is proposed for UV mapping purpose.

PHEBUS is a double spectrometer for the EUV (Extreme Ultraviolet) range (55-155 nm) and the FUV (Far Ultraviolet) range (145-315 nm) dedicated to the characterization of Mercury's exosphere composition and dynamics, and surface-exosphere connections. The optical configuration of PHEBUS can be divided into two independent parts. The collecting part consists of a straylight rejection baffle, an off axis parabolic mirror and an entrance slit, allowing to scan Mercury's exosphere thanks to a rotating mechanism. This movable mirror collects the light from the exosphere above thelimb and focuses it to the entrance slit. The parameters of the mirror were calculated so as to have a 170 mm effective focal length and a folding angle of 100º. This part determines the FOV (Field of View) and the LOS (Light of Sight). The spectrometer part determines the spectral resolution of the instrument and is composed by the entrance slit, two holographic gratings and the detectors. 58) 59) 60)

The spectrum detection is based on the photon counting method and is done using MCP (Micro-Channel Plate) detectors with RAE (Resistive Anode Encoder). Photocathodes are CsI for the EUV range, and CsTe for the FUV range. The size of the detectors active area is 40 x 25 mm2 equivalent to a matrix of 1024 x 512 virtual pixels (spectral x spatial). Furthermore Calcium and Potassium lines are selected by the FUV grating. These extra visible lines are monitored using PM (Photomultipliers) with bialkali photocathode also used in photon counting mode.

The main advantage of the MCP+RAE detectors is their very high sensitivity mainly due to a very low dark current. Thus photon counting is easily achievable on typical experiment temperature range (from -20ºC to+40ºC), avoiding mass and power expensive devices to cool the detectors. Five to six orders of magnitude for the detection are then a typical value and offer the monitoring of a wide range of emission.

Optical specifications: The wavelength ranges are 55-155 nm for the EUV and 145-315 nm for the FUV. The spectral resolution is defined in terms of FWHM (Full Width at Half Maximum) and Full Width at 1% of maximum (FW1%). The required spectral resolution is 1 nm for EUV and 1.5 nm for FUV. These values are to be compared with the result of the optical design optimization: the FWHM is about 0.5 nm on EUV, and 0.8 nm on FUV. Furthermore, the FW1% is about 0.9 nm on EUV, and 1.5 nm on FUV. These calculated values do not include any spreading effects due to scattering by gratings.

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Figure 38: a: PHEBUS instrument, b: Optical configuration 1 – Entrance pupil; 2 – Entrance mirror; 3 – Slit; 4 – FUV grating; 5 – EUV grating; 6 – FUV detector; 7 – EUV detector (image credit: LATMOS/CNRS)

 

SERENA (Search for Exosphere Refilling and Emitted Neutral Abundance)

SERENA will study the gaseous interaction between Mercury's surface, exosphere, magnetosphere and the solar wind. PI: S. Orsini, CNR/IFSI (Institute of Space Astrophysics and Planetology), Rome, Italy. The instrument has been developed by an international team: SwRI (San Antonio, TX, USA); Space Research Institute (Austrian Academy of Sciences, Graz, Austria); IRF (Swedish Institute of Space Physics), Kiruna, Sweden; Physicalisches Insitut (Space Research & Planetary Sciences, University, Bern, Switzerland); FMI (Finnish Meteorological Institute), Helsinki, Finland; MPS (Max Planck Institute for Solar System Research), Kathlenburg-Lindau, Germany.

The SERENA instrument suite will study in-situ the composition, the vertical structure and the source of the deposit processes of the exosphere of Mercury. Such an environment is composed by thermal and directional neutral atoms (exosphere) originating via surface release and charge-exchange processes, and by ionized particles originated through photo-ionization and again by surface release processes. In order to accomplish the scientific goals, in-situ analysis of the environmental elements is necessary, and for such a purpose the SERENA instrument shall include four units: 61) 62) 63)

• two Neutral Particle Analyzers:ELENA and STROFIO

• two Ion Spectrometers (MIPA and PICAM).

In particular, ELENA investigates the neutral gas escaping from the surface of Mercury, and the related involved processes; STROFIO investigates the exospheric gas composition; PICAM investigates the exo-ionosphere extension and composition, and the close-to-planet magnetospheric dynamics; MIPA investigates the plasma precipitation toward the surface and ions energized and transported throughout the environment of Mercury.

The main scientific goals of SERENA are:

- Chemical and elemental composition of the exosphere

- Exo-ionosphere composition and distribution

- Surface emission rate and release processes

- Plasma precipitation rate

- Particle loss rate from Mercury's environment

- Gas density profile asymmetries

 

ELENA (Emitted Low-Energy Neutral Atoms)

The ELENA instrument within the SERENA suite is a new kind of low energetic neutral atoms instrument, devoted to detect neutral atoms from E ~20 eV up to E~5 keV, within a nadir pointing 1-D FOV (perpendicular to the spacecraft orbital plane).

The main ELENA scientific objectives are as follows: 64)

- Surface emission rate and release processes

- Particle loss rate from Mercury's environment

- Remote sensing of the surface properties

- ENA imaging applications for comparative solar-planetary relationship.

ELENA is a TOF (Time-of-Flight) sensor, based on the state-of-the- art of ultra-sonic oscillating shutter (operated at frequencies above 20 kHz and up to 50 kHz), mechanical gratings and MCP (Micro-Channel Plate) detectors. The purpose of the shuttering system is to digitize space and time when tagging the incoming particles without introducing "disturbing" detector elements, which may affect the particle's trajectory or the energy. This is particularly important in this case, in which neutrals of energies of a few tens of eVs must be detected.

Energy range

<0.02-5 keV (mass dependent)

Velocity resolution (ΔV/V)

Down to 10%

Viewing angle

4.5º x 76º

Angular resolution

4.5º x 4.5º (actual), 4.5º x 2.4º (nominal pixel)

Mass resolution M/ΔM

H and heavy species

Optimal temporal resolution

40 s

Geometric factor G

2 x 10-5 cm2 sr

Integral geometric factor

6 x 10-4 cm2 sr

Table 6: Performance parameters of ELENA

The ELENA sensor concept is showed in the Figure 39. The entrance of the start section (an aperture of about 1 cm2) allows the impinging neutral particles to enter through the shuttering system with a definite timing. Particles are then flown into a TOF chamber, and finally detected by a 1-dimensional array composed by MCP and a discrete set of anodes corresponding to a FOV of 4.5° x 76°, allowing the reconstruction of both, velocity and direction of the incoming particle events. The spacecraft footprint track will provide the second dimension.

BepiColombo_Auto24

Figure 39: Photo of the ELENA EQM instrument (image credit: CNT/IF SI)

The composite radiation made by neutrals, ions and light impinges onto the ELENA sensor entrance. A grid system placed between the main ELENA entrance and the shutter minimizes the ion and electron background radiation before the shuttering system. The internal ion deflector is a stack of particle cross-track grids which introduce a transversal E-field able to filter out the bulk of the charge particles of both signs.

 

STROFIO (Start from a Rotating FIeld mass spectrOmeter)

The STROFIO instrument within the SERENA suite is a mass spectrograph that determines particle mass-per-charge (m/q) by the TOF technique. The name comes from the Greek word "Strofi", which means "to rotate": the phase of a rotating electric field "stamps" a start time on the particles' trajectory, and the detector records the stop time.

STROFIO is the only instrument that can detect all neutral species reaching the spacecraft, not just those with emissions or absorptions at specific wavelengths. In contrast with optical techniques, STROFIO can make measurements with high sensitivity, both in full sunlight and in Mercury's shadow. The main science objectives of the STROFIO instrument are the following:

- Chemical and elemental composition of the exosphere and its variability

- Neutral gas density asymmetries

- Surface release processes

- Relationship between the exospheric composition and the surface composition.

STROFIO is characterized by a high-sensitivity (0.14 counts/s when the density is 1 particle/cm3). The achieved mass resolution is m/Dm = 60. STROFIO is a novel type of mass spectrometer: the start time is imprinted on the trajectory of the particle by a radio frequency electric field, that bends the trajectory in a given plane, and the stop time is the time when the particle reaches the detector.

In particular, the neutral particles enter into the ionization chamber through the entrance in the ram direction (see the figure). The neutral gas is ionized and accelerated into the mass analyzer. Here the ions experience the effects of an electric field, constant in magnitude, but with direction rotating uniformly in space, in a plane perpendicular to the initial ion velocity, at a frequency f. The trajectory of an ion can hit the detector only if the field points to the detector while the ion traverses the dispersing region. At other times, the ion will simply miss the detector. The time difference between the instant when the particle arrives at the detector and the time when the field was pointing in the appropriate direction is equal to the travel time through the field free region. In the table the major STROFIO characteristics are summarized.

BepiColombo_Auto23

Figure 40: Illustration of the STROFIO instrument (image credit: CNR/IFSI)

Energy range

< 1 eV

Viewing angle

20º

Mass resolution M/ΔM

60

Mass range

1-64 dalton amu (atomic mass unit)

Sensitivity

0.14 (counts/s / (particles/cm3)

Temporal resolution

10 s

Table 7: Performance parameters of STROFIO


MIPA (Miniature Ion Precipitation Analyzer)

MIPA is a simple ion mass analyzer optimized to provide monitoring of the precipitating ions using as little spacecraft resources as possible. The analyzer is capable to measure all main groups of ions present in the magnetosphere. The energy range and mass range of the analyzer is optimized to cover accelerated ionospheric ions. The main science objectives of MIPA are the following: 65)

- Plasma precipitation rate

- Planetary response to SW (Solar Wind) variations

- Magnetosphere structure and dynamics.

The ion flux arrival angle and energy are analyzed by an electrostatic deflector, comprising of two 90° cylindrical electrodes, followed by an 128° double focusing cylinder electrostatic analyzer. The ions exiting the energy analyzer are post accelerated up to 1 keV energy by a voltage applied to a TOF cell. Inside the cell, ions hit START and STOP surfaces producing secondary electrons recorded by two ceramic channel electron multipliers giving respective timing. For energies above 4 keV, the post acceleration is switched off. The timing of the event gives the ion velocity and, in combination with known energy, the mass. The Figure shows the MIPA principle elements and the overall view. The MIPA total G-factor can be controlled/decreased by decreasing the post acceleration voltage resulting in lowering the impact energy and thus secondary electron yield from the START and STOP surfaces, and reducing the size of the aperture slits. The table summarizes the MIPA major characteristics.

Energy range

15 eV - 15 keV

Energy resolution ΔE/E

7%

FOV (Field of View)

90º x 360º, 4 polar x 6 azimuth pixels

Angular resolution FWHM (Full Width Half Maximum)

22.5º x 60º (polar x azimuth)

Mass range

1-50 amu (atomic mass unit)

Mass resolution

M/ΔM ~5

Sampling time

7.8125 ms

Time resolution

18 s, 4 polar x 6 azimuth x 96 energy steps

Efficiency

1-10% (adjustable to decrease geometrical factor)

Geometrical factor

~10-5 cm2 sr eV / eV per pixel, w/efficiency

Table 8: Performance parameters of MIPA

BepiColombo_Auto22

Figure 41: Photo of the MIPA instrument (image credit: CNR/IFSI)

 

PICAM (Planetary Ion CAMera)

PICAM operates as an all-sky camera for charged particles, allowing the determination of the 3D velocity distribution and mass spectrum for ions from thermal up to ~3 keV energies and in a mass range extending up to ~ 132 amu (Xenon). The instantaneous 2p FOV coupled with this mass range and a mass resolution better than ~ 100 is a unique capability, which provides to PICAM superior performances in the frame of the MPO mission. The major PICAM scientific objectives are the following: 66)

- Exo-ionosphere composition and distribution

- Planetary response to SW variations

- Magnetosphere structure and dynamics.

Figure 42shows a general layout of the sensor. The ion optics is based on the principle of a modified pinhole camera. The sensor is symmetric along the Z-axis and its FOV is a hemisphere centered along this axis. Ions enter through an annular slit (figure). After reflection on an ellipsoidal ion mirror (2) the 90° polar angle distribution is folded into a 15° angular range. Here the ions pass a modulated wire gate which defines discrete packets of ions for analysis of the time-of-flight until the particles impact on the MCP. The modulation can be either single-shot or with a pseudo-random sequence which results in higher efficiency. Particles pass through a narrow, toroidal analyzer and through an exit slit and are reflected by a planar mirror. After energy selection in the toroidal analyzer, they enter the TOF and imaging section. A cross section of the ion optics is shown in the figure. UV rejection will be obtained by a striated primary mirror covered by a non-reflecting layer of Cu2S, which decreases the UV reflection by a factor of 1000. Multiple reflections within the instrument, the small entrance slit and the narrow exit slit in front of the mass analyzer provide very strong protection. The outer part of the ion optics is designed for hot conditions. The lower part of the sensor containing the MCPs and the detector electronics is thermally decoupled. The table summarizes the PICAM major characteristics.

BepiColombo_Auto21

Figure 42: Photo of the PICAM device (image credit: CNR/IFSI)

Energy range, energy resolution ΔE/E

1 eV -3 keV, 7%

Viewing angle

3-D, 1.5p

Angular resolution

~22.5º

Mass resolution M/ΔM

>60

Mass range

1 -~132 amu (Xe)

Time resolution

1-32 s

Geometric factor G = SW (Solar Wind)

2.3 x 10-2 cm2 sr

Geometric factor ΔE/E

1.6 x 10-3 cm2 sr eV/eV

Table 9: Performance parameters of PICAM

 

SYMBIO-SYS (Spectrometers and Imagers for MPO BepiColombo Integrated Observatory System)

SYMBIO-SYS, also written as SIMBIO-SYS, will examine (also in stereo and color) Mercury's surface geology, volcanism, global tectonics, surface age and composition, and geophysics. PI: E. Flamini, ASI, Italy. SIMBIO-SYS, is an international project led by Italy, with main cooperation from France and Switzerland. The STC instrument is a collaboration of CNR-IFN UOS Padova,CISAS University of Padova,, INAF -Osservatorio Astronomico di Padova, Selex-Galileo, University of Padova, and ASI. 67) 68) 69)

SIMBIOSYS incorporates capabilities to perform 50 - 200 m spatial resolution global mapping in both stereo mode and color imaging, high spatial resolution imaging (5 m/px scale factor at periherm) in panchromatic and broad-band filters, and imaging spectroscopy in the 400 - 2200 nm spectral range. This global performance is reached using three independent channels:

• STC (STereoscopic imaging Channel)

• HRIC (High Resolution Imaging Channel)

• VIHI (Visible and near-Infrared Hyperspectral Imager).

BepiColombo_Auto20

Figure 43: a) SYMBIO-SYS STC stereo configuration; b) position and size of useful filter strips images on the detector area (in black), image credit: SYMBIO-SYS team)

STC (STereoscopic imaging Channel) optical design:

STC is a double wide angle camera designed to image each portion of the Mercury surface from two different perspectives, providing panchromatic stereo image pairs required for reconstructing the DTM (Digital Terrain Model) of the planet's surface. In addition, it has the capability of imaging some portion of the planet in four different spectral bands (Figure 43).

The STC design is composed of two "sub-channels" looking at the desired stereo angles, that share the majority of the optical elements and the detector (Figure 44). With respect to classical two- or single-camera designs, this solution allows to reach good stereo performance with general compactness, saving of mass, volume and power resources.

In general, stereo cameras adopt a pushbroom acquisition mode: the detector is a linear array and the full bidimensional image is reconstructed placing side by side each of the lines successively acquired at a suitable rate determined by the spacecraft velocity. For STC, instead, a push-frame mode has been chosen; in this case the detector is a CMOS APS (Active Pixel Sensor) bidimensional array, so actual 2D images of the planet surface are acquired, then buffered and read while the spacecraft moves. Only when the image on the detector has shifted along track by an amount corresponding to the FoV of each filter, another image is acquired.

The selected APS device has the snapshot option, that is substantially an electronic shutter; for this reason, no mechanical shutter has been foreseen for this instrument. The push-frame acquisition method allows to have some overlap of the imaged regions in the along-track direction, increasing the image matching accuracy and taking into account possible small drifts of the satellite pointing.

STC scientific requirements

Scale factor

50 m/px at periherm

Swathwidth

40 km at periherm

Stereoscopic properties

± 21.4° stereo angle with respect to nadir; both images on the same detector

Vertical accuracy

80 m

EE (Encircled Energy)
MTF (Modulation Transfer Function)

> 70% inside 1 pixel
> 60% at Nyquist frequency

Wavelength coverage

410-930 nm (5 filters)

Filters

Panchromatic (700±100 nm), 420±10 nm, 550±10 nm, 750±10 nm, 920±10 nm

STC optical characteristics

Optical concept

Catadrioptic: modified Schmidt telescope plus folding mirrors fore-optics

Stereo solution (concept)

2 identical optical channels; detector and most of the optical elements common to both channels

Focal length (on-axis)

95 mm

Pupil size (diameter)

15 mm

Focal ratio

f/6.3

Mean image scale

21.7 arcsec/pixel (105 µrad/pixel)

FOV (cross-track)

5.3º

FOV (along-track)

2.4º panchromatic, 0.4º color filters

Detector

Si_PIN (format: 2048 x 2048; 10 µm squared pixel); 14 bit dynamic range

Table 10: STC scientific requirements and STC optical characteristics

BepiColombo_Auto1F

Figure 44: The entire STC optical design is shown: in (a) the configuration is viewed in the plane defined by the along-track and nadir directions; in (b) the projection in the orthogonal plane, the one including across-track and nadir directions, is given. In the inset, an enlarged view of the focal plane region helps to better follow the rays which are focalized on the APS detector (image credit: SYMBIO-SYS team)

 

HRIC (High Resolution Imaging Channel): 70) 71)

The accommodation of the HRIC channel in the SYMBIO-SYS experiment is shown in Figure 45. The HRIC optical design has been developed at INAF Astronomical Observatory of Capodimonte, with the main objective of characterizing relevant Mercury surface features at very high spatial resolution (pixel scale of about 5 m at 400 km from planet surface) in the visible range. The high resolution images of selected regions will allow the project to identify key surface features (e.g., craters, scarps, lava flows and plains) and to study their relation with internal processes, as well as the effect of external agents, such as meteor bombardment. In addition, HRIC images will be of paramount importance in support of experiments aiming to the identification of Mercury orbital parameters, such as the obliquity and the amplitude of the libration.

The HRIC optical design has been optimized in order to satisfy not only scientific requirements and nominal optical performance, but also dimensional requirements and mechanics constraints of compactness and low mass required for space applications. The optical design is based on a catadioptric concept, with optimization of a Ritchey-Chretien configuration by a dedicated corrector (Figure 46). The instrument has a focal length of 800 mm and is equipped with a dedicated refractive camera, in order to correct the field of view covered by a detector of 2 k x 2 k pixels, with a pixel size of 10 µm. The focal ratio of the instrument is f/8, in order to be diffraction limited at 400 nm and to optimize radiometric flux and overall mechanical dimensions. The main HRIC optical characteristics are reported in Table 12 . The combined (reflective + refractive) solution guarantees a good balance of achieved optical performances and optimization of resources (mainly volume and mass). The curvature of the lenses is spherical, in order to simplify manufacturing.

The quality of the optical design has been optimized and checked by analysis with the Zemax code. The adopted configuration corrects and transmits well over the whole band of observation (400-900 nm). A relative obscuration ratio of 0.3 (between primary and secondary mirror diameters) has been achieved in order to provide a good energy transfer to the telescope exit pupil. The optical design presented in this paper takes also into account the foreseen filters and detector package, in which the detector window acts also as filter.

BepiColombo_Auto1E

Figure 45: SYMBIO-SYS instrument including the three integrated channels: HRIC (High Resolution Imaging Channel), STC (STereoscopic imaging Channel) and VIHI (Visual and near-Infrared Hyperspectral Imager), image credit: SYMBIO-SYS team 72)

The main tasks of the HRIC are to provide high resolution images of selected Mercury surface features like craters, scarps, lava flows and plains with a panchromatic filter and to help in geo-mineralogical characterization of local surface features by band-pass filters.

Pixel scale

5 m/pixel at periherm (400 km from Mercury surface)

Pixel resolution

12.5 rad/pixel (with pixel of 10 µm)

Spectral range

400-900 nm

Image quality

Diffraction limited at 400 nm

Filters

1 panchromatic (650 nm central wavelength, 500 nm bandwidth),
3 band-pass (550 nm, 700 nm, 880 nm, 40 nm bandwidth)

Table 11: HRIC main scientific requirements

BepiColombo_Auto1D

Figure 46: HRIC optical layout (SYMBIO-SYS team)

Optical configuration

Catadioptric: Ritchey-Chretien modified with a dedicated corrector

Aperture

100 mm

Angular FOV (Field of View)

1.47

F-number

8

Focal length

800 mm

Image scale

12.5 µrad/pixel

Focal plane detector

2048 x 2048 (CMOS APS)

Detector pixel size

10 µm x 10 µm

Table 12: HRIC optical parameters

Since the system is diffraction limited, the fraction of diffraction EE (Encircled Energy) curves enclosed in one pixel and the diffraction MTF (Modulation Transfer Function) until the Nyquist frequency have been considered in order to evaluate the image quality. For both diffraction EE and MTF, central obscuration has been included. In addition, the RMS spot radius on the image plane and field curvature and distortion have been evaluated. The field corrector has been optimized in order that images in different filter bands can be compared without distortion and field curvature. Thus, the image quality is high over the entire field of view. The main optical performances are reported in Table 13.

Polychromatic diffraction EE in one pixel

70%

Polychromatic MTF at Nyquist frequency

59%

Polychromatic RMS spot diameter (geometric)

0.8 µm

Field curvature

12 µm

Distortion

0.05%

Table 13: HRIC main optical performances

 

VIHI (Visible and near-Infrared Hyperspectral Imager)

VIHI is one of the three optical channels of the SYMBIO-SYS instrument suite for the BepiColombo mission to Mercury. Its scientific objective is to study the hermean surfaces composition by sensing the photon flux reflected off the planet. VIHI works in the spectral range of 400 -2000 nm with 256 spectral channels (6.25 nm/band sampling). The particularity of this channel is the use of a single detector matrix (264 x 264) for both visible and infrared wavelengths.

The instrument has an instrument FOV (Field of View) of 250 µrad corresponding to a spatial scale of about 100 m/pixel at periherm and 375 m at apoherm. The instrument operates in pushbroom configuration, sampling the surface of Mercury with an FOV of 64 x 0.25 mrad. The main technical challenges of this experiment are the focal-plane design (cadmium-mercury-telluride thinned to improve the efficiency at visible wavelengths), the short dwell time (from about 40 ms at the equator to about 100 ms at the poles), thermal control, mechanical miniaturization, radiation hardening, high data rate, and compression. 73)

 

SIXS (Solar Intensity X-ray Spectrometer)

SIXS will perform continuous measurements of X-rays and particles of solar origin employing a very wide field of view. PI: J. Huovelin, Observatory University of Helsinki, Finland.

The objective of SIXS on MPO (Mercury Planetary Orbiter) is to investigate the direct solar X-rays, and energetic protons and electrons which pass the spacecraft on their way to the surface of Mercury. These measurements are vitally important for understanding quantitatively the processes that make Mercury's surface glow in X-rays, since all X-rays from Mercury are due to interactions of the surface with incoming highly energetic photons and space particles. The X-ray emission of Mercury's surface will be analyzed to understand its structure and composition. SIXS data will also be utilized for studies of the solar X-ray corona, flares, solar energetic particles, and the magnetosphere of Mercury, and for providing information on solar eruptions to other BepiColombo instruments. 74) 75)

SIXS consists of two detector subsystems. The X-ray detector system includes three identical GaAs PIN detectors which measure the solar spectrum at 1–20 keV energy range, and their combined field-of-view covers ~1/4 of the whole sky. The particle detector system consists of an assembly including a cubic central CsI(Tl) scintillator detector with five of its six surfaces covered by a thin Si detector, which together perform low-resolution particle spectroscopy with a rough angular resolution over a field-of-view covering ~1/4 of the whole sky. The energy range of detected particle spectra is 0.1–3 MeV for electrons and 1–30 MeV for protons.

A major task for the SIXS instrument is the measurement of solar X-rays on the dayside of Mercury's surface to enable modeling of X-ray fluorescence and scattering on the planet's surface. Since highly energetic particles are expected to also induce a significant amount of X-ray emission via PIXE (Particle-Induced X-ray Emission) and bremsstrahlung when they are absorbed by the solid surface of the planet Mercury, SIXS performs measurements of fluxes and spectra of protons and electrons. SIXS performs particle measurement at all orbital phases of the MPO as the particle radiation can occur also on the night side of Mercury.

The energy ranges, resolutions, and timings of X-ray and particle measurements by SIXS have been adjusted to match with the requirements for interpretation of data from Mercury's surface, to be performed by utilizing the data of the MIXS (Mercury Imaging X-ray Spectrometer), which will measure X-ray emission from the surface.

BepiColombo_Auto1C

Figure 47: Illustration of the SIXS instrument elements (image credit: SIXS team)

Measurement principle: SIXS is capable of performing measurements of X-ray spectra with time resolution down to 1 second in the energy range 1- 20 keV, and simultaneous proton and electron spectra in the energy range 0.33–30 MeV for protons and 50 keV–3 MeV for electrons. Both X-ray and particle channels are capable of measuring count-rates up to 20,000 cps with very small pile-up, and have a low background. Both channels have a total FOV of at least 180º in diameter. The detectors can be operated in near room temperature, and the detector materials are highly radiation tolerant. The instrument has on-board radioactive sources (Fe55) for spectral calibration of the X-ray detectors.

Science data will be made available in PDS, FITS, and partly in ASCII formats. Scientific analysis of the data will be carried out primarily at the home (PI) institutes of the SIXS and MIXS instruments. Standard science analysis will include determination of the physical energy scale and spectral resolution of the X-ray data using background signal from the Fe55 source, and fitting of the X-ray spectra with publicly available astronomical X-ray spectrum analysis software. The X-ray standard analysis results will be further interpreted for purposes of fluorescence analysis of Hermean surface by MIXS, and also for independent studies of X-ray corona and flares of the Sun.

 



MMO (Mercury Magnetospheric Orbiter) Spacecraft

The MMO spacecraft mainly aims the study of the magnetic field and magnetosphere of Mercury. JAXA/ISAS is responsible for its development and operation on the Mercury orbit. 76)

Earth and Mercury each have an intrinsic magnetic field, but Mars and Venus do not. Why? Comparison of the Mercury's magnetic field with Earth's will bring us closer to an understanding of the terrestrial magnetic field and magnetosphere, and the magnetospheres on the various scales in the Universe.

The main science objectives of the MMO spacecraft observations are to study the following phenomena:

• Structure and origin of Hermean magnetic field: The magnetic field around Mercury in high accuracy will be surveyed for the study of the origin of planetary magnetic field.

• Structure, dynamics, and physical processes of Hermean magnetosphere: The magnetosphere will be surveyed in detail for the investigation of the universality and the singularity of planetary magnetospheres.

• Structure, variation, and origin of Hermean exosphere: The large-scale structure and variation of the thin 'atmosphere' tell us their generation / disappearance processes.

• Environment of inner solar system: The powerful environment near the sun is observed, and the energy process is solved.

MMO is a spin-stabilized spacecraft with a spin rate of 4 seconds. The objective is to observe the 3-dimensional velocity distribution of particles. A sufficient centrifical force is needed for the extension of four long wire antennas (15 m) and two MASTs (5 m) in support of electromagnetic field measurements. 77)

The spin axis is almost perpendicular to the Mercury equatorial plane. This serves two requirements: 1) the rejection of the solar reflection light into the upper / lower surface of the spacecraft and 2) to be able to point the HGA (High-Gain Antenna) toward Earth with small efforts. The harsh environment near Mercury (0.31AU from the Sun at perihelion) imposes 11 solar constant irradiation on the MMO spacecraft, while its thermal control system is required to maintain the onboard equipment and the spacecraft structure in a proper temperature range during the entire mission phases.

The MMO is controlled by means of passive thermal design techniques and some components are controlled by means of a combination of passive and active techniques. The passive thermal control elements are the SSM (Second Surface Mirror), the thermal shield, paints, films, and MLI (Multi Layer Insulation) blankets. All external surfaces should be electrical conductive due to scientific requirements.

Spacecraft: The MMO has an octagonal shape, which can be surrounded by a 1.8 m diameter circle (Figure 48). The height of the side panel is 1.06 m. The instruments are located on the upper and lower decks with 0.4 m in between them. The HGA (High Gain Antenna) is a helical array antenna of 0.8 m diameter. The HGA is pointed towards the Earth by the ADM (Antenna Despun Motor) and an elevation control mechanism APM (Antenna Pointing Mechanism). ADM and APM are protected from direct solar illumination by the ‘APM sunshade,' which is made of MLI. The MGA (Medium Gain Antenna) RX (Receiver), a bi-reflector type antenna, is mounted on the lower surface with an extensible mechanism. Most of the scientific instruments (particle sensors, etc.) are allocated on the lower deck and look out of the side panels through cut-outs, while two pairs of probe antennas for plasma wave instruments and one pair of extensible booms for magnetometers are installed on the outside. The scientific instruments looking out of side panels are covered by a dedicated sun-shield to prevent direct solar heat input as much as possible. The extendible booms for magnetometers are covered by a single layer Germanium-coated black Kapton to avoid direct solar illumination to the MMO interior through the cut-outs for the booms.

The HGA-APM is an elevation over azimuth mechanism with hollow actuators which house dual rotary joints for X- and Ka- band. Dry lubricated CDA gearhead motors with a 120:1 ratio drive the mechanism. 78)

BepiColombo_Auto1B

Figure 48: Illustration of the MMO spacecraft (image credit: JAXA/ISAS)

 

MOC (Molecular Contaminant) source materials on MMO (Ref. 77):

The main source materials of MOC which release the majority of outgas were selected according to the CVCMs (Collected Volatile Condensable Materials) and weights of the materials used in MMO. The project selected four main source materials; an epoxy adhesive Hysol® EA9396, a silicone adhesive RTV-S691, a silicone adhesive ELASTOSIL®-S692, and a silicone transfer adhesive TRANS-SIL®. Hysol® EA9396 is used in the solar cell panels. RTV-S691 is used to bond solar-cells and as the thermal filler. ELASTOSIL®-S692 is used to bond SSMs. TRANS-SIL® is used in MLI and the extensible booms.

The thermal environment of MMO is different before and after the separation from MOSIF: The MMO is low in temperature and not illuminated by sunlight before the separation, while it is high in temperature and illuminated after the separation. Since the outgassing characteristics depend on temperature and the MOC accumulation rates depend on temperature and the intensity of sunlight,the MOC accumulation problem is differently treated by the project.

 

Orbit: Aftermission arrival at Mercury, the MMO spacecraft willbe separated by spin-ejection and enter to into a polar elliptical orbit of Mercury with "400 km periherm and 12,000 km apoherm", in order to provide magnetic field mapping of all spheres and observation of Mercury's magnetosphere in all regions. The orbital period is 9.2 hours. The MMO and MPO orbits are coplanar. However, in one MMO orbit, the MPO, in a lower elliptical orbit with a period of 2.3 hours, will orbit Mercury 4 times.

BepiColombo_Auto1A

Figure 49: Artist's rendition of the MMO spacecraft in Mercury orbit (image credit: JAXA)

 

MMO (Mercury Magnetospheric Orbiter) development status:

• April 29, 2015: The MMO FM (Flight Model), Japan's contribution to the joint BepiColombo mission, arrived at ESTEC in Noordwijk, the Netherlands on April 20. The spacecraft was unpacked at ESA's Test Center and will be submitted to follow-on mechanical testing of the complete stack, known as the MCS (Mercury Composite Spacecraft). 79) 80) 81)

- MMO will sit at the top of the BepiColombo stack on launch in January 2017. It will be placed atop ESA's MPO (Mercury Planetary Orbiter), which will be attached in turn to a carrier spacecraft, the MTM (Mercury Transfer Module), tasked with transporting the other two via highly efficient electric propulsion.

BepiColombo_Auto19

Figure 50: Photo of the MMO spacecraft at ESA/ESTEC (image credit: ESA, A. Le Floc'h)

• March 19, 2015: The MMO was shown to the media on March 15. The Mercury Exploration Mission "BepiColomobo" is now under development with a target launch for JFY 2016. The MMO will be shipped to Europe soon to be placed in the BepiColombo. 82)

• Nov. 1, 2014: The thermal vacuum test for the Mercury Magnetosphere Orbiter (MMO) of the BepiColombo was performed for three weeks at JAXA Sagamihara Campus. The photos show the transportation of the MMO from a cleanroom to the thermal-vacuum chamber (Ref. 82).

BepiColombo_Auto18

Figure 51: The photo shows the transportation of the MMO from a cleanroom to the thermal-vacuum chamber (image credit: JAXA)

 


 

MMO sensor complement: (MERMAG, MPPE, PWI, MSASI, MDM)

MMO will carry five advanced scientific experiments that will also be provided by nationally funded PIs (Principal Investigators), one European and four from Japan. Significant European contributions are also being made to the Japanese instruments. 83)

MERMAG (Mercury Magnetometer)/MGF

The MMO's MERMAG, also referred to as MGF (Magnetic Field Investigation), will provide a detailed description of Mercury's magnetosphere and of its interaction with the planetary magnetic field and the solar wind. PI: W. Baumjohann, Austrian Academy of Sciences, Austria. For MERMAG-M, the lead institution is IWF (Institut für Weltraumforschung) in Graz. The primary objective of the magnetic field investigation on MMO is to study the formation and dynamics of Mercury's magnetosphere and the processes that control the interaction of the magnetosphere with the solar wind and with the planet itself. Emphasis will be placed on those effects and processes which are particular to the Hermean magnetosphere and distinguish it from the better known terrestrial one: (1) the weakness of the field, (2) the small dimension and possible greater importance of kinetic effects, and (3) the near-absence of an ionosphere. It is expected that these differences have a large impact on (a) the reconnection process, both on the dayside and in the magnetotail, (b) the structure and dynamics of field-aligned currents, and (c) the low frequency plasma waves.

 

MPPE (Mercury Plasma Particle Experiment)

MPPE will study low- and high-energetic particles in the magnetosphere. The MPPE consortium is led by the PI (Principal Investigator) Y. Saito, ISAS, JAXA, Japan. 84) 85)

The scientific objectives of the experiment are: Structure, dynamics, and physical processes of the Hermean magnetosphere The magnetosphere will be surveyed in detail to investigate the dynamics of planetary magnetospheres in close proximity to the Sun. 86)

- Formation and characteristics of the small-scale magnetosphere

- Solar wind contribution to the magnetospheric plasmas

- Stability of the plasma sheet

- Substorms at Mercury

- Particle acceleration, trapping, and loss.

MPPE is a comprehensive instrument package for plasma, high-energy particle and energetic neutral atom measurements. It consists of seven sensors. The first six sensors perform in-situ observations and cover the charged particle species and the energy range of interest from the space plasma physics point of view.

• For the electrons: MEA (Mercury Electron Analyzer): Electron Spectrometer for energies 3 eV-30 keV. There are 2 devices, MEA1 and MEA2 mounted 90º apart for highresolution coverage. And HEP-e (High Energy Particles - electron): Electron-Spectrometer for energies 30-700 keV and a FOV (Field of View) of 20º x 130 º.

• For the ions: MIA (Mercury Ion Analyzer): Ion-Spectrometer for energies 5 eV/q to 30 keV/q without mass resolution. MSA (Mass Spectrum Analyzer) and HEP-ion to measure the TOF (Time of Flight) ion spectrometer for ions with energies of 5 eV/q to 40 keV/q, mass range 1 to 60 amu and angular resolution 10º-15º.

ENA (Energetic Neutral Atom): Detector for energetic neutral atoms with a mass resolution at 10 eV of 3.3 keV and an angular resolution of 9º x 30º. ENA will detect energetic neutrals created via charge-exchange and will provide remote information on how plasma and neutral gas interacts in the Hermean environment.

Since comprehensive full three-dimensional simultaneous measurements of low to high-energy ions and electrons around Mercury as well as measurements of energetic neutral atoms will not be realized before BepiColombo/MMO's arrival at Mercury, it is expected that many unresolved problems concerning the Mercury magnetosphere will be elucidated by the MPPE observation.

The interaction between the solar wind and Mercury's magnetosphere is unique for many reasons. For example, due to the closeness of the sun and the weak intrinsic magnetic field of Mercury, its magnetosphere is severely compressed; the estimated distance between the subsolar magnetopause and the surface is less than half the planetary radius. This could mean that sometimes the solar wind can directly interact with the planetary surface. Even if this doesn't happen, the plasma at the cusp can always interact with the surface, since there is no strong mirror reflection due to the smallness of the mirror ratio. How this kind of direct interaction with the surface affects the rest of the magnetospheric processes is an intriguing question that can never be tested in other planetary magnetospheres.

 

PWI (Mercury Plasma Wave Instrument)

PWI will make a detailed analysis of the structure and dynamics of the magnetosphere. PI: Y. Kasaba, RISH, University of Kyoto, Japan. PWI will provide remote-sensing observations of electric fields, plasma waves, and radio waves in the Hermean magnetosphere and exosphere.

The PWI consists of three sets of receivers EWO (Electric Waveform Onboard), SORBET (Spectroscopie Ondes Radio & Bruit Electrostatique Thermique), and AM2P (Active Measurement of Mercury's Plasma), connected to two sets of electric field sensors MEFISTO (Mercury Electric Diled In-Situ Tool) and WPT (Wire-Probe anTenna) and two kinds of magnetic field sensors LF-SC (Low Frequency Search Coils), and DB-SC (Dual-Band Search Coil).

 

MSASI (Mercury Sodium Atmospheric Spectral Imager)

MSASI will measure the abundance, distribution and dynamics of sodium in Mercury's exosphere. PI: Ichiro Yoshikawa, University of Tokyo, Japan. The main objectives of MSASI are: 87)

1) To image the sodium exosphere (column density and spatial distribution) with a spatial resolution of 1/64 RM and intensity range from 10 M to 10 K Rayleigh. These are sufficient to find sodium rich spots on the surface, if they do exist (e.g. radar bright spots and Caloris basin).

2) To measure local and temporal variations of the sodium exosphere (time scale: less than a few hours)

- short-term: Substorm

- middle: North/South asymmetry

- long: Seasonal variation and places of interest (Caloris basin, polar regions, etc.)

3) To identify release mechanisms which are stimulated by incoming materials such as solar wind ions and micro-meteoroids.

MSASI) is a high-spectral, high-temporal resolution, and wide-FOV imager. The spectral resolution is 80,000 (λ/Δλ) and it is enough to separate sodium emission from bright surface reflection of Mercury and to observe dayside exosphere. The temporal resolution is 2 ms, which enables to achieve the spatial resolution of 1.25 km on the MMO (Mercury Magnetospheric Orbiter), which is a spin stabilized spacecraft. Additionally, MSASI has a moving mirror to change its field of view. The moving mirror enables to scan full Mercury in 80 seconds. To achieve high temporal resolution, a high speed CMOS image sensor and an image intensifier are used in the detector unit which is assembled in Japan. To achieve high spectral resolution, a Fabry-Perot etalon and a temperature-stable narrow band filter are used in the optics which is assembled in the UK. To achieve a wide FOV, a moving mirror unit is assembled in Russia. The critical design review was successfully finished and manufacture of PFM was started. 88)

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Figure 52: Illustration of the MSASI instrument (image credit: University of Tokyo)

 

MDM (Mercury Dust Monitor)

MDM will study the distribution of interplanetary dust in the orbit of Mercury. PI: Hiromi Shibata, Kyoto University, Japan.

BepiColombo MMO is a spinstabilized spacecraft as the Helios spacecraft and the MDM will be installed on the side panel to face an in-plane direction of ecliptic similar to the ecliptic sensor of the Helios Micrometeoroid Analyzers. The MDM uses four plate sensors of piezoelectric lead zirconate titanate (PZT) because it has (1) a simple configuration, (2) a large sensitive area compared with the mass of the system (220 g for the sensor part, 381 g for the electronics), (3) a high-temperature tolerance up to +230 °C, and (4) no bias voltage needed. 89)

The MDM has a sensitive area of 64 cm2 with an open aperture and the nominal observation is for 1 year. Thus, the MDM is predicted to attain a similar count rate of interplanetary dust to the Helios Micrometeoroid Analyzer, although it depends on the lower limit of detectable mass range.

In contrast to Helios, the BepiColombo MMO will be in an elliptic orbit around Mercury with the perihermion of 400 km and the aphermion of 11824 km. The orbital inclination is 90º, the orbital period is 9.3 h. Thus, the Mercury-centric dependence of dust flux will be investigated, near the apoherm the MDM can predominantly detect dust particles from interplanetary space, while near periherm it may detect dust particles launched from the Mercury surface by meteoroid bombardments or by an electrostatic force near the terminator as suggested also in the lunar dust environment.

Science significance of dust observation in Mercurial orbit: The goal of the MDM is the detection of Mercury ambient dust particles and new insight into the environment of dust particles in the inner solar system.

Dust sciences related to Mercury. The incoming dust particles to Mercury are related to the space weathering of the surface materials, the origin of Mercury's atmosphere, and dust particles of Mercurial origin. Micrometeoroid impact might contribute to the formation of the tenuous Na atmosphere and the space weathering effect can constrain the chronology of the Mercurial surface. For those purposes, the observation of temporal and directional variations in the dust influx throughout Mercurial orbit is important and it leads an estimation of the external mass accretion rate to the Mercurial surface.

The outgoing dust particles from Mercury are related to Mercurial dust ejection by meteoroid impacts, similar to the Jovian satellites and also levitating dust. Those dust particles possibly have interaction with the magnetic field, similar to the Jovian dust streams.

Dust sciences of the inner solar system. In addition to Keplerian interplanetary dust (IPD) that slowly spirals to the Sun by the Poynting-Robertson effect, the project considers other dust components: dust trails of near-Sun comets, β meteoroids, and interstellar dust (ISD) around Mercury's orbit.

Dust

Origin

Kinematic properties

ISD (Interstellar Dust)

Interstellar medium

25 km/s, ecliptic longitude of 252º & ecliptic latitude of 5º

IPD (Interplanetary Dust)

Cometary and asteroidal materials

< 77 km/s Anti-apex of the Mercury or higher eccentricities and various inclinations

β meteoroid

Interplanetary dust swept by solar radiation

> 77 km/s, Solar direction

Ejecta cloud

Materials on the Mercury surface, regolith

> 0 km/s, Mercury direction

Levitating dust

Materials on the Mercury surface, regolith

> 0 km/s, Mercury direction

Table 14: Properties of dust particles detectable in the orbit around Mercury

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Figure 53: BepiColombo MMO schedule as of June 2015 (image credit: JAXA/ISAS) 90)

 


 

MOSIF (MMO SunShield and InterFace Structure)

The MOSIF acronym very much describes its functionality, which then leads to its configuration. The MOSIF consists of two major assemblies:

• The Adapter has at its center the circular inter-face to the MMO, the arms provide the attachments for the Sunshield, the arms provide the 4 separable attachments to the MPO

• The Sunshield structure supports the MLI which shades the MMO. The Sunshield is truncated at 18° to allow MCS/MOSIF tilting towards the sun (for navigation purposes) without illuminating the MMO. The Sunshield is conical at 16° half-angle to tolerate wobble of the MMO during its slowly spinning separation.

The MOSIF carries the electrical interfaces to between the MPO and the MMO and after MMO separation retains the MMO spin-ejection system which has by then been fired by the MPO.

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Figure 54: MOSIF - with MMO installed (image credit: ESA, Airbus DS)

 

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MTM (Mercury Transfer Module)

The MPO and MMO scientific spacecraft are brought to Mercury by the MTM, containing propulsion and power systems for the cruise phase. While the MPO and MMO designs are optimized for operation in Mercury orbit, the MTM provides pre-Mercury functions. The MCS (Mercury Composite Spacecraft) cruise configuration is progressively disassembled approaching the arrival at Mercury by in-flight separations until the 2 orbiters are operational in their respective orbits.

During the electric propulsion thrust phases the MCS is orientated with the thrusters pointing in the flight direction to provide braking and with thruster steering supporting the attitude control system. During coast phases the MCS is stabilized by rotation around the sun vector, thereby minimizing chemical propellant consumption.

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Figure 55: Cross-section through the MCS configuration (image credit: Airbus DS)

The MTM is at the base of the 'stack' and provides propulsion for Earth-Mercury transfer and to slow down its approach to Mercury. It carries no scientific instruments. The Transfer Module is equipped with two propulsion systems: a standard CPS (Chemical Propulsion System) which is bipropellant using MMH/MON3. The CPS, operating in blowdown mode, is used for navigation and AOCS support during the cruise.

The transfer phase is accomplished by an electrical propulsion system within the MTM, comprising 4 steerable Xenon-fuelled ion thrusters which operate either singly or in pairs. The power generation and distribution hardware needed during Cruise, generating more than 11 kW, resides in the MTM.

Power subsystem: The MTM provides a 100 V regulated bus for the electric propulsion system and a 28 V regulated bus for other equipment plus the MPO/MMO. The MTM includes a 12 Ah lithium-ion battery for damping of MEPS surge current (in the event of a beam-out) and also provides power during the short eclipses. The solar array provides 13 kW which is particularly driven by the 2 x 5 kW demand of the MEPS (Ref.17) .

The MTM solar array is a high temperature design operating at maximum 190°C which uses the same technologies as the MPO, but without the need for in-plane thermally conductive CFRP facesheets. The thermal control of the array is achieved by the AOCS which tilts the array to a maximum of 76° from the sun. The size of the array is driven by the need to provide maximum power also at 0.31 AU. While approaching the sun the solar array output initially increases, of course accompanied by an increase of temperature. Once the temperature has reached 190°C (at about 0.5 AU) the array must be tilted, thereby reducing its projected area and limiting its output. The two wings total 40 m2 and have a mass of 290 kg (Ref. 22).

MEPS (MTM Electric Propulsion System):

The MEPS contains the 4 electric propulsion (Kaufmann) thrusters, their power processing electronics, 4 thruster pointing mechanisms and the xenon storage and feed system. The thrusters are the QinetiQ T6, Ø 22 cm, 145 mN thruster derived from the T5 flown on the GOCE mission. The system is planned to operate over 25 thrust arcs totalling 880 days, with the longest continuous operation being for 167 days. The system typically operates using two thrusters. The MEPS will be inactive for at least 30 days prior to each fly-by in order to provide a quiet period for precise orbit determination from the ground.

The thrusters are mounted on individual pointing mechanisms which enable the thrust vector to point through the MCS center of mass – either directly for a single thruster or by the plane of an operating pair of thrusters. By off-pointing of the thrust vector a moment is created for use by the AOCS for wheel off-loading. The xenon system, with its flow control units, pressure regulators and 3 tanks, is able to store and deliver 580 kg of xenon which can provide a ΔV of 5400 m/s. 91)

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Figure 56: Photo of the QinetiQ T6 thruster in a firing test (image credit: QinetiQ)

The MEPS is controlled by the central platform on-board software. The AOCS has a specific EPCM (Electric Propulsion Control Mode) for MEPS operations. In EPCM, the AOCS is also capable of performing reaction wheel offloading by offpointing the thrust vector. - For the April 2018 trajectory, 22 thrust arcs are planned with a total duration of 756 days, and the longest arc lasting about 150 days. See Figure 21 for the distribution of thrust arcs throughout cruise (Ref. 38).

Interplanetary navigation for a spacecraft employing electric propulsion is significantly different from missions relying on chemical propulsion only. The pre-launch trajectory planning by mission analysis (shown in Figure 21) is relying on conservative assumptions on the available thrust level, in turn dependent on the maximum power available from the MTM solar array. If the available power in-flight should turn out to be higher, corresponding higher thrust levels will be used to shorten a thrust arc, reducing its criticality. This will be assessed by the operations teams prior to each thrust arc, and revised on a weekly basis during a thrust arc, with the Flight Control Team providing the latest figures on the power available to Flight Dynamics for planning of the thrust arc.

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Figure 57: Photo of the MTM (Mercury Transfer Module) thruster floor at Thales Alenia Space, Turin, Italy (image credit: ESA)

Legend to Figure 57: The BepiColombo PFM (Proto Flight Model) of the MTM thruster floor is shown upon delivery to Thales Alenia Space in Turin for testing. The two-axis pointing systems for the four thrusters are installed; the thrusters themselves will be integrated while the MTM is at ESTEC, along with the rest of the flight model hardware for the electrical propulsion system. 92)

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Figure 58: Beam-out transient at Ion thruster (image credit: Airbus DS)

The effects are two-fold: on one hand, they are present on the PPU secondary side, potentially affecting equipment located close to the thruster, or having harness running in parallel to the thruster PPU connections; on the other hand, they result in transients on the PPU supply lines. The latter have been applied to the MTM PCDU during unit level tests, to verify that the quality of the 100 V power bus is not adversely affected.

Impact of the transients on the thruster side is minimized by the use of shielded high voltage cables throughout, which are, moreover, routed separately from all other S/C harness. A critical area, however, is at the thruster interface, where the thruster pointing mechanism is located in close vicinity. These mechanisms allow two-axis steering of the ion thrusters, featuring sensitive encoders and sensors. To check electromagnetic compatibility, a model of the thruster harness was implemented in the pointing mechanism qualification model, and a dedicated transient test performed injecting worst-case transients + appropriate margin into the thruster harness. The impact on the sensor signal integrity was then measured and demonstrates that no mechanism step loss will occur if a beam-out transient happens during mechanism operation.

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Figure 59: Pointing mechanism test set-up (image credit: Airbus DS)

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Figure 60: MEPS configuration on the MTM (image credit: ESA)

CPS (Chemical Propulsion Systems): The MTM CPS employs redundant 12 x 10 N bipropellant thrusters, using MMH (Mono-Methyl Hydrazine) and MON (Mixed Oxides of Nitrogen). This system is derived from Eurostar 3000 systems. As well as attitude control capability, the MTM CPS can provide axial thrust for cruise navigation. 157 kg of propellant are carried, giving a capability of 68 m/s ΔV plus attitude control.

 

Transmitter/Receiver EMC: BepiColombo Command & Control is performed by an X-band system for Earth to S/C commanding and S/C to ground data transmission. For the science phase, this system is supplemented by a Ka-band downlink for payload data transmission, not used in cruise phase. Consequently, the spacecraft emissions must be closely controlled in the sensitive X-band receiver band around 7167 MHz, while large emissions are generated at the transmit frequencies of 8420 MHz and 32 GHz. Moreover, there is a payload instrument featuring a Ka-band receiver at 34.4 GHz which is, however, in general not used during cruise phase.

The communication links are using a suite of 4 antennas, comprising two LGAs (Low Gain Antennas) at opposite spacecraft sides (used for the LEOP phase), a steerable MGA (Medium Gain Antenna, as the main antenna in cruise), and a steerable HGA (High Gain Antenna -as the nominal antenna for science operation, allowing optimized antenna pointing to maximize the link budget). While the LGAs and MGA are transmitting/receiving in X-band only, the HGA provides communication in both X- and Ka-band.

Calculation of electric fields from MGA and HGA requires the use of simulation programs which fully consider the near-field behavior taking into account antenna patterns & polarization, and spacecraft geometry. For BepiColombo, the commercial software CST Microwave Studio® has been used. The algorithm for the solution of the scattering problem is using the so called MLFMM (Multilevel Fast Multipole Method) based on the well-known MOM (Method of Moments), which is used to calculate the surface current distribution by generating and solving an appropriate linear equation system. A recursive solving scheme is used for the efficient final solution of the numerical problem, making the MLFMM algorithm a very effective numerical tool for the calculation of electromagnetic field solutions for large objects. The resulting spatial current distribution can then be used to analytically calculate the electromagnetic fields on the surface of the structure as well as the far field.

While the external fields from X-band and Ka-band transmitters can reach very high levels depending on location and antenna orientation, the spacecraft-internal equipment is shielded against these large E-fields to a certain extent, mainly by the spacecraft MLI (Multilayer Insulation). The spacecraft design is thermally extremely demanding, the satellite being subjected to tremendous thermal fluxes, from both the sun and the Mercury surface. This can only be survived by a special MLI design which features more than 5 times the number of aluminum layers compared to a standard earth-orbiting spacecraft, e.g. TerraSAR, with the side effect that a shielding efficiency of 23 dB can easily be assumed in the X- and Ka-band frequency ranges (backed up by measurements from other projects, which show a shielding efficiency of 30 dB already for standard design). This allows limiting the RS E-field requirement for internal equipment and instruments to 32 V/m which is within the standard capabilities of EMC test houses, and can easily be complied with for most equipment.

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Figure 61: MMO orbiter illumination by HGA fields during the cruise phase (MMO S/C hidden behind sun shield), image credit: Airbus DS

External equipment being subjected to extreme E-fields had to undergo dedicated equipment level RS (Remote Sensing) tests to prove compatibility with the antenna emissions. In a similar manner the impact of S/C equipment in the receiver bands has been assessed, resulting in a specification of E-field levels allowable at RX frequencies, depending on external equipment location.

The reason of maintaining a low DC magnetic moment, however, is ultimately only an aid to minimize the low frequency AC magnetic emissions of the MPO module in order to ensure proper Gradiometer performance, taking into account the instrument measurement principle. The major task was thus to identify any potentially AC critical unit, and test these at an early stage to judge the potential impact on the MERMAG performance. The main contributors are:

- Strong fluctuating currents (mainly in power units)

- Mechanisms with rotating magnetic elements [mainly motors, e.g. flywheels, SADM (Solar Array Drive Mechanism), Antenna Pointing Mechanism]

- Instruments with similar mechanisms (PHEBUS instrument baffle motor, MERTIS instrument pointing unit)

- Parts which change their magnetic performance during flight, depending e.g. on temperature. A number of MLI mounting parts have been identified to show such behavior, which could be significantly reduced by parts demagnetization prior to MLI manufacturing.

AC emission testing of critical units was performed in the frame of EM module EMC testing (sample Figure 62: one axis of PHEBUS baffle motor). The tests revealed that the PHEBUS baffle rotating reference magnets generate strong AC fields during baffle operation, requiring the installation of compensation magnets at opposite locations, which was then later performed on the flight instrument.

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Figure 62: PHEBUS baffle magnetic field of the X-axis, uncompensated (image credit: Airbus DS)

Ultimately an AC magnetic characterization will be performed on the integrated MPO satellite FM, in the frequency range 0-200 Hz. The satellite will be configured and operated in the most flight representative manner possible, operating all AC magnetic critical equipment in parallel. A differential magnetometer will be accommodated in a location representative for the MERMAG instrument. Using a gradiometer allows to perform this test in ambient clean room environment, as long as the external magnetic environment can be kept stable during the test in a limited area of only of 2 – 3 m around the spacecraft (Ref. 92).

 


 

EMC (Electromagnetic Compatibility) testing of the BepiColombo Spacecraft

Generally it is the aim of a system EMC test to verify electromagnetic compatibility of the entire spacecraft, by operating the complete satellite in most emissive mode and measuring the power bus quality as well as verifying proper system performance. 93)

- On BepiColombo spacecraft, system tests are split into two parts, first operating the power subsystem of the transfer module together with the electric propulsion running in a special vacuum chamber, and then, running the entire spacecraft with suitable thruster simulators.

- The BepiColombo spacecraft faces two problems using this approach: During cruise to Mercury, it comprises effectively three spacecraft mounted in a stack configuration, and, more importantly, it features an Ion Propulsion thruster subsystem which consumes some 70% of the entire stack power, while being able to be operated only in a special vacuum chamber, which is not compatible with the set-up of the completed stack, nor with standard EMC test facilities operating at ambient pressure, or even standard vacuum chambers.

1) Power subsystem test: For this unit level test, the power subsystem of the transfer module was brought to the thruster manufacturer where it was operated together with the Ion thruster in a dedicated vacuum chamber. On stack level the thruster itself will be replaced by a suitable simulator powered by the thruster power conditioning unit. A crucial point of the first test step was thus to demonstrate the representativeness of the thruster simulator comparing relevant electrical parameters to measurements obtained with the Ion thruster running in vacuum.

2) Spacecraft EMC testing in a second step: Due to the modularity of BepiColombo, Spacecraft level EMC testing comprises several configurations:

a) MPO orbiter EMC: this covers the EMC behavior of the complete MPO S/C operating in Mercury orbiting mode. The full suite of payload instruments is operated in this configuration.

b) Stack level EMC testing: this comprises the cruise configuration of MPO (and Japanese orbiter MMO). While instruments are off during cruise, this includes the MTM ion propulsion with twin thruster operation as worst case.

MPO spacecraft EMC tests have been performed in 2015 in two dedicated test campaigns at ESTEC laboratories, for conducted and radiated EMC. The tests covered launch mode operations and, more importantly, science operation with all scientific instruments.

In the first campaign, performance of the MPO main bus was checked under full load. All spacecraft units and instruments were operated in mission mode to characterize the bus voltage ripple in both frequency and time domain. Particular emphasis was placed on transient operation, as there are mechanisms, reaction wheels, and a pulsed laser accommodated in one instrument.

Radiated EMC testing was performed in the ESTEC "Maxwell" anechoic chamber, and additionally involved operation of all spacecraft antennas, in

- emissive mode; in particular, to measure antenna radiation towards sensitive instruments, and in

- receiving mode; to verify spacecraft emissions in receiver notches and demonstrate that spacecraft RF uplink is not degraded by spacecraft emissions from any equipment including all instruments.

The remaining verification step is the future Stack level EMC test, Figure. 63. This test will comprise a Conducted EMC test with 2 SEPS (Solar Electric Propulsion System) operating in twin thruster mode, i.e. PPUs powering simulated thrusters. The test will finally demonstrate power bus quality under max. power consumption, including bus ripple and performance of the power link powering the MPO orbiter spacecraft. Conducted emissions (i.e. voltage ripple) will be measured on:

- MTM main power bus 100 V, and 28 V bus

- 60V link (MTM supply of MPO spacecraft during cruise)

- 28V main bus in the MPO spacecraft in steady state, in both Frequency and Time Domain.

As the most demanding case, simulated beam-outs will be performed on one thruster branch, short circuiting high voltage within the thruster simulator. During the event, simultaneous measurements of voltage transients on all power busses will be performed, as well as currents on all SEPS supply lines, to check the impact of a beam-out on the entire spacecraft stack including the second thruster branch. During this test the instruments will be off as they are not operated during the cruise to Mercury. — Nevertheless, this will be the final (and most demanding) test to verify Electromagnetic Compatibility of the full BepiColombo spacecraft suite.

Note: The MTM (Mercury Transfer Module) PCDU (Power Control and Distribution Unit) provides MEPS (MTM Electric Propulsion System) plus chemical propulsion (for cruise AOCS and navigation correction) and it provides power for electric propulsion system and for MPO +MMO. The electric propulsion system is called SEPS (Solar Electric Propulsion System), consisting of two PPUs (Power Processing Units) and four ion thrusters, called SEPT (Solar Electric Propulsion Thrusters), of which two are operated in parallel, the others providing redundancy. Since the ion thrusters generate a high energy beam of Xe Ions, thrusters operation can only be performed in special vacuum chambers featuring a graphite target. Thus for the majority of on-ground test, SEPT thrusters must be replaced by PPU load simulators.

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Figure 63: Spacecraft level Conducted EMC test configuration (complete stack with MPO and MMO), image credit: ESA

 

On-ground measurement of time-varying magnet fields on-board BepiColombo's MPO spacecraft

The time-varying magnetic fields generated on ESA's BepiColombo MPO (Mercury Planetary Orbiter) spacecraft have been measured recently to assess their influence on the Mercury magnetometer (MERMAG) instrument. The BepiColombo MPO spacecraft will travel to Mercury for extensive scientific investigations carrying various platform equipment and 12 scientific payload instruments. During the operation of these various units their magnetic fields will vary over time and depending on their strength the fields may be observed as spurious signals by MERMAG. The time-varying magnetic fields generated by the spacecraft are therefore measured on ground and compared to the design goal of maximum 3.2 nT under the guidance of a Magnetic Review Board composed of members from MERMAG, industry, and ESA. 94)

After some preliminary measurements on the MPO ETB (Electrical Test Bench) at Airbus Defence and Space in Friedrichshafen in 2012, the actual MPO PFM (Proto-Flight Model) spacecraft has been measured in January 2016 in a dedicated campaign in the test center at the ESTEC (European Space Research and Technology Center). The MPO PFM spacecraft was operated with all platform equipment and payload instruments powered on, including power conditioning and distribution unit, several heaters of the thermal control system and a representative thruster valve of the chemical propulsion system. Moreover, mechanisms active during scientific mission phases were exercised, e.g. reaction wheels for attitude and orbit control, antenna pointing mechanisms of the high-gain antenna, and the SADM (Solar Array Drive Mechanism).

Test setup: The test equipment consists of two sets of two magnetometer sensors denoted as MAG1 and MAG2 and a digital E-Box (Electronics Box), Figure 65.

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Figure 64: The two sensors of the Differential Magnetometer Measurement Set-Up in vicinity of SADM in upper right corner (image credit: ESA)

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Figure 65: Differential magnetometer measurement set-up (image credit: ESA)

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Figure 66: Differential magnetometer positions, viewed from z-side (zenith direction). The stationary set is at a location similar to the deployed MERMAG instrument, the other set is movable to various locations (image credit: ESA)

In each set the magnetometer sensors are separated, for example by a distance d = 60 cm. Such a set of two magnetometers is sometimes referred to as gradiometer, because it allows to calculate an approximation to the magnetic field gradient along the separation baseline between the two sensors. For the measurements, however, the project used the magnetic field measurements themselves and also their difference, i.e. it will be used rather as differential magnetometer. The sensors are commercial equivalents of the actual MERMAG sensors. Both E-Boxes are connected to the data acquisition system, which records and displays the data during the measurements.

The magnetometer sensors are digitally controlled and allow synchronous sampling of all three axes from both magnetometers with a timing error less than 1 µs. The used sampling frequency of 200 Hz allows to reject emissions from the power utility network, as it is operated at the European standard frequency of 50 Hz. This allows in principle measurements up to about 100 Hz. The actual MERMAG instrument will sample at 128 Hz, i.e. it will measure up to 64 Hz.

One differential magnetometer set was located stationary close to the in-flight location of the deployed MERMAG instrument, which is mounted on a short boom (Figure 66). The outboard sensor of the stationary set was placed therefore at the in-flight location of the deployed inboard sensor. The other set was positioned at various locations close to the MPO spacecraft to perform measurements in the vicinity of platform equipment and payload instruments. Measurements closer to the spacecraft increase the signal-to-noise ratio of possible interference emissions, which then can be extrapolated to the in-flight location of the MERMAG instrument by distance scaling techniques.

The magnetometer coordinate system was setup parallel to the spacecraft coordinate system, but with different orientations (Figure 66).

Test results of SADM:

At the location of the SADM, the distance of both magnetometer sensors from the spacecraft was approximately 20 cm and the closest of the two sensor was at about 70 cm from the SADM. In Figure 67 the acquired data of the time series is shown for all three axes, which is however already smoothed by a sliding average over 20 data points.

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Figure 67: Time series of the measured differential signal for all three magnetometer axes (image credit: ESA)

The two activations of the drive mechanism at about 1000 s and 1500 s are clearly visible. They correspond to a full rotation in counter clockwise direction with the first activation rotating 180° and the second activation rotating another 180° in the same direction to the initial position. This is the main rotation and directly related to the solar array angle.

However, they are amplitude modulated by another signal. This has been isolated and low-pass filtered via an additional sliding average of 50 data points, i.e. the data was effectively band-pass filtered. As shown in Figure 68 it has a time period of about 57 s, which could be related to the internal gear box or the motor currents.

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Figure 68: Amplitude modulation signal during the first activation shown in Figure 67 (image credit: ESA)

The amplitude of the major rotation and the amplitude modulation are both roughly 20 nT. This would correspond to approximately 0.18 nT in about 4.8 m distance in case of a cubic distance law, which is typical for magnetic moments. The actual MERMAG instrument, however, is positioned in another direction. Therefore, distance scaling cannot be applied without reservations, but the stationary differential magnetometer close to the flight-representative MERMAG location had apparently not recorded an excessive magnetic field during the operation.

Along the magnetometer y-axis, the signal was low and indicates the presence of additional periodic signals with lower amplitudes at higher frequencies, possibly also related to the internal gearbox or the motor currents.

The dynamic spectra reveal the presence of signals with frequencies of about 3.5 Hz, 10 Hz, 13.5 Hz, and 17 Hz, which are all correlated to the SADM operation.

Summary: While the magnetic fields seem to be within the required limits, some unexpected periodic variations have been detected with frequencies above the fundamental rotation frequency of the solar array, which are under assessment to check correlation with mechanism and drive electronics properties. Additional results will be discussed in future publications.

Among the other scientific instruments PHEBUS (Probing of Hermean Exosphere by Ultraviolet Spectroscopy) is of special interest, because it has an external, moving baffle, which uses a magnet for position control. During its instrument development a compensation magnet has been installed with the goal to cancel the magnetic field at the MERMAG in-flight location. Moreover, the pointing mechanism of MERTIS (Mercury Thermal Infrared Spectrometer) and the high-intensity laser of BELA (BepiColombo Laser Altimeter) are possible sources of time-varying magnetic fields.

These measurements on BepiColombo MPO will provide valuable experience and lessons learned also for several other projects and upcoming missions such as Solar Orbiter, JUICE (Jupiter Icy Moons Explorer), the ESA M4 mission candidate THOR (Turbulence Heating Observer), or the joint ESA-CAS (Chinese Academy of Sciences) mission candidate SMILE (Solar Wind Ionosphere Magnetosphere Link Explorer), which also have ambitious magnetic cleanliness requirements on time-varying fields originating from the spacecraft platform equipment and other payload instruments.

 

EMC measurements of the PICAM (Planetary Ion Camera), a part of the SERENA instrument suite aboard BepiColombo

SERENA (Search for Exospheric Refilling and Emitted Natural Abundances) is a particle detection suite on MPO comprising four complementary instruments, among them the PICAM, which provide complete composition analysis of the Hermean plasma envelope. The PICAM characteristics is shown in Table 9. The remaining three instruments are ELENA (Emitted Low Energy Neutral Atoms), STROFIO (Start from a Rotating Field Mass Spectrometer), and MIPA (Miniature Ion Precipitation Analyzer). 95)

During mission phase there will be high thermal stress caused by the solar constant at Mercury (perihelion value ~ 14.5 kW/m2) and thermal radiation from the planet, in addition ~ 3 kW/m2 for BepiColombo altitude. PICAM top parts get temperatures up to 225 °C, the MCP (Micro channel Plate) has ~ 70 °C, the electronics box is below 50 °C,. Since the design phase these requirements are best possible considered for the PICAM instrument development, including the impact on the EMC performance.

The structure of PICAM is a crucial point regarding the EMC performance, the FM box is shown in Figure 69. The ion optics – right part – consists of (starting from the ion entrance following the trajectory) primary mirror, gate, deflecting electrodes, converging lens, toroidal analyzer, secondary mirror, retarding grids, repeller grids, and backvoltage MCP. The five electronics boards – left part – are the detector, gate encoder and driver, high voltage supply, controller, and low voltage DC/DC converter. This setting (including grounding) is of particular importance if CE (Conducted Emission) and susceptibility is considered, because the interface is through the SCU (System Control Unit) board of the ELENA box, Figure 70 top panel. The other SERENA instruments have their connections also at that unit (precaution to prevent cross coupling) to the S/C.

BepiColombo_Auto6

Figure 69: Photo of the PICAM FM (Flight Model), image credit: ESA

BepiColombo_Auto5

Figure 70: Grounding and structure induced noise setup (image credit: ESA)

EMC Test Procedure: The PICAM instrument is fully qualified according the BepiColombo EMC requirements ECSS (European Cooperation for Space Standardization), Electromagnetic Compatibility, the list includes: (i) grounding and isolation, (ii) bonding, (iii) conducted emission, (iv) conducted susceptibility, (v) radiated emission, (vi) radiated susceptibility, and (vii) electrostatic discharge.

EMC Results: In Figure 71, the FM (Flight Model) and QM (Qualification Model) is compared in a laboratory setup.

Laboratory Setup: The detailed test configuration is show in Figure 70. Top: PICAM mass spectrometer grounding diagram including the primary power (28 V) connector J01 and the space wire telemetry/telecommand I/F connector J02. Three units are relevant, (i) the S/C power distribution, (ii) the SCU board in the ELENA box and from there (power and space wire) (iii) to the PICAM instrument with several internal subunits. The wiring, internal grounding and connection to the S/C structure is shown. Bottom: Laboratory setting including the full harness configuration and grounding schema for the structure induced noise measurements (analogue schema for power line and telemetry measurements).

Figure 71 shows CE/CM measurements for the FM (blue trace) and QM (green), the limit lines are in red color. 1st-top panel: PICAM in Hadamard mode (25 ns gate timing, gate voltage 15 V), emissions of the power lines, frequency range 10 kHz – 50 MHz, bandwidth (BW) 1 kHz. The emissions in the upper frequency range, in particular the narrow 40 MHz line, are significantly reduced. Second bottom panel: Bonding strap emissions, instruments in Hadamard mode (25 ns, 15 V), BW 1 kHz. The emissions of the FM are generally below the QM, in particular the two narrow band violations of the QM are reduced to ~ 25 dBμA, the 40 MHz peak (Hadamard 25 ns mode) is removed.

The FM current consumption in the various modes is: {standby, single pulse, Hadamard (25, 50, 100, 200) ns} = {92, 132, (180, 150, 137, 128)} mA.

BepiColombo_Auto4

Figure 71: PICAM FM and QM conducted emission (image credit: ESA)

Power Line Emissions: The power line emissions are a crucial point regarding interference due to direct connection to the SCU board. Figure 72 shows the FM CE/CM measurements in four different configurations in the frequency range 10 kHz – 50 MHz, BW settings are 1 kHz (below 150 kHz) and 10 kHz (150 kHz – 50 MHz).

Legend to Figure 72:

1st-top panel: instrument power off with a permanently present 75 kHz analyzer disturbance, not related to PICAM. 2nd panel: standby mode with broadband emissions in the frequency range 8 – 10 MHz related to the FPGA and telemetry. 3rd panel: Hadamard option with gate voltage 15 V and 25 ns gate timing. Characteristic is the 96 kHz peak with an amplitude of 27 dB µA well below the 60 dB µA limit line boarder. 4th-bottom panel: single pulse mode with 50 V gate voltage and 25 ns timing. The typical 40 MHz peak has an amplitude slightly below the limit (30 dB µA).

 

BepiColombo_Auto3

Figure 72: PICAM power line emissions, common mode (image credit: ESA)

Bonding Strap Emissions: The setup for bonding strap measurements is shown in Figure 70 (bottom panel), results are depicted in Figure 73.

- The top panel shows the bonding strap emissions (blue line) in the frequency range 20 Hz – 100 kHz, PICAM is in Hadamard mode with 25 ns gate timing and 15 V gate voltage (red line is the limit, yellow and green lines are probe specific curves). Visible in the lab setup are the mains frequency (50 Hz) and related uneven harmonics, e.g. {3rd, 5th, 7th} = {150, 250, 350} Hz, PICAM related emissions are not present.

- Second center panel: CE/CM measurements on the bonding strap, Hadamard mode with 25 ns gate timing, 15 V gate voltage. Again visible is the 96 kHz Hadamard peak and a related broadband emission around 15 MHz. The BW is 1 kHz in the frequency range 10 kHz – 150 kHz and 10 kHz in the range 150 kHz – 50 MHz; violations of the limit at the lower frequency end are due to the selected BW setting.

- Third bottom panel: background emission on the bonding strap, same BW as in 2nd panel.

BepiColombo_Auto2

Figure 73: Structure noise emissions, common mode (image credit: ESA)

Hadamard Options: Figure 74 shows two Hadamard options, gate timing of 25 ns (blue line) and 200 ns (black line), both 15 V gate voltage, analyzer BW 1 kHz. Characteristic for the 25 ns timing is the 96 kHz peak, for the 200 ns timing option a 25 dB µA 40 MHz emission is visible.

BepiColombo_Auto1

Figure 74: Hadamard mode ({25, 200} ns) emissions (image credit: ESA)

PICAM performance improvements:

Figure 75 shows the azimuth and elevation mapping of an incoming ion beam (He+, 500 eV) entering the center of sector 4 with 55° elevation angle (top panel) and 85° elevation (bottom panel). For the FM (right column of the panels) very good results of target hits are achieved. The FM has a slightly modified mirror design, the measured overall performance, visible in the beam mapping, is significantly improved. An improved thermal design of the FM gate driver electronics via copper band is an additional instrument internal EMC relevant grounding contact to the electronics box structure.

BepiColombo_Auto0

Figure 75: Ion beam (He+, 500 eV) mapping (image credit: ESA)

 



BepiColombo Ground Segment

The two spacecraft will be independently operated and controlled from the MPO OGS (Operations Ground Segment), located at ESOC (European Space Operations Center) and the MMO Operations Center,SSOC (Sagamihara Space Operations Center) at JAXA/ISAS. The science operations for MMO will be prepared by the MMO SSOC , while the MPO science operations by the MPO SGS (Science Ground Segment), located at ESAC (European Space Astronomy Center) in Madrid, Spain. 96)

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77) Hiroyuki Ogawa, Fumitaka Urayama, Susumu Baba, Eiji Miyazaki, Akira Okamoto, Hajime Hayakawa, “Assessment of molecular contamination on the BepiColombo MMO spacecraft,” 44th ICES ( International Conference on Environmental Systems), Tucson, Arizona, 13-17 July 2014, paper: ICES-2014-283, URL:
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78) Pablo Campo, Aingeru Barrio, Fernando Martin, ”Testing of BepiColombo Antenna Pointing Mechanism,” Proceedings of the 16th European Space Mechanisms and Tribology Symposium 2015, Bilbao, Spain, 23–25 September 2015 (ESA SP-737, September 2015)

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83) “MMO Instruments,” CNES, URL: http://smsc.cnes.fr/BEPICOLOMBO/GP_instr_mmo.htm

84) Y. Saito, J.A. Sauvaud, M. Hirahara, S. Barabash, D. Delcourt, T. Takashima, K. Asamura, “Scientific objectives and instrumentation of Mercury Plasma Particle Experiment (MPPE) onboard MMO,” Planetary and Space Science, Volume 58, Issues 1–2, January 2010, pp: 182–200

85) “MPPE - Mercury Plasma/Particle Experiment,” ESA, URL: http://www.cosmos.esa.int/web/bepicolombo/mppe

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87) Ichiro Yoshikawa, Oleg Korablev, Shoichi Okano, “MSASI -Mercury Sodium Atmospheric Spectral Imager,” URL: http://sirius.bu.edu/mercury/workshop2/ParisMSASI.ppt

88) Shingo Kameda, Go Murakami, Ichiro Yoshikawa, Oleg Korablev, David Rees, “The Mercury Sodium Atmosphere Spectral Imager (MSASI) onboard BepiColombo/MMO,” 38th COSPAR Scientific Assembly, Bremen, Germany, July 15-18, 2010, URLof abstract: http://adsabs.harvard.edu/abs/2010cosp...38..746K

89) M. Kobayashi, H. Shibata, K. Nogami, M. Fujii, T. Miyachi, H. Ohashi, S. Sasaki, T. Iwai, M. Hattori, H. Kimura, T. Hirai, S. Takechi, H. Yano, S. Hasegawa, R. Srama, E. Grün, “Dust observation in Mercurial orbit by Mercury Dust Monitor of BepiColombo,” 44th Lunar and Planetary Science Conference (2013), The Woodlands, TX, USA, March 18-22, 2013, URL: http://www.lpi.usra.edu/meetings/lpsc2013/pdf/2172.pdf

90) ”BepiColombo Mercury Exploration,” JAXA, 2015, URL: http://www.stp.isas.jaxa.jp/mercury/index-e.html

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93) K. Kempkens, ”System EMC Qualification: The Incremental Approach – The BepiColombo Power Subsystem with Ion Propulsion,” Proceedings of the 2016 ESA Workshop on Aerospace EMC(Electromagnetic Compatibility), Valencia, Spain, May 23-25, 2016, (ESA SP-738, May 2016)

94) A. Junge, A. Przyklenk, H.-U. Auster, D. Heyner , L. A. D’Arcio, K. Kempkens, ”On-ground measurement of time-varying magnet fields on-board BepiColombo's MPO spacecraft from a Solar Array Drive Mechanism,” Proceedings of the 2016 ESA Workshop on Aerospace EMC(Electromagnetic Compatibility), Valencia, Spain, May 23-25, 2016, (ESA SP-738, May 2016)

95) H. Eichelberger , G. Fremuth, G. Prattes, Ch. Kürbisch, G. Laky, F. Giner, S. Neukirchner, R. Wallner, H. Jeszenszky, M. Leichtfried, J.-J. Berthelier , K. Torkar , H. I. M. Lichtenegger, ”BepiColombo-MPO-SERENA-PICAM EMC measurements,” Proceedings of the 2016 ESA Workshop on Aerospace EMC(Electromagnetic Compatibility), Valencia, Spain, May 23-25, 2016, (ESA SP-738, May 2016)

96) Sara de la Fuente, Jonathan McAuliffe, Mauro Casale, ”Science Operations Planning Concept for BepiColombo Mercury Planetary Orbiter,” Proceedings of the 14th International Conference on Space Operations (SpaceOps 2016), Daejeon, Korea, May 16-20, 2016, paper: AIAA 2016-2591, URL: http://arc.aiaa.org/doi/pdf/10.2514/6.2016-2591


The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).

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